Let me begin with the in-film question, asked by Samuel L. Jackson's character, that sets up the entire plot of the movie and gives the film its eponymous title: "So here's the riddle. What does an eight thousand pound mako shark with a brain the size of a flat head V8 engine and no natural predators think about?" Thomas Jane's character provides the answer, and consequently the movie's name, toward the end as he stares at the facility's fencing: "Those fences are titanium underneath, but on top they're just plain steel. They've been herding us, pushing us where they want, using us to flood the facility. That's the answer to the riddle. Because that's what an 8,000 pound mako thinks about. About freedom. About the deep blue sea."
Look, guys, what more can I say about this horribly-executed movie that hasn't already been said? There are so many issues with this movie that a person could write ten books or film a 5-hour long documentary detailing them and the terrible decisions behind them, and that still wouldn't be enough to sufficiently offer probable causes that could satisfy the appalled audience.
Firstly, and most importantly, the sharks featured here are NOT mako sharks. I know that they're supposed to be much larger and much more deadly than regular sharks because of the supposedly explained "gene therapies to increase their brain mass" because "a larger brain means more protein". Even so, these sharks here are NOT mako sharks. They're definitely something, but they're NOT mako sharks. They're also not any type of shark, extant or not. The shortfin mako shark, which is what the filmmakers are trying to emulate in the physique here, are nowhere nearly that large. They're no more than 4 meters (13 feet) in length. Their counterparts, the longfin makos, are so-called because their pectoral fins can be as long as or longer than their heads. Longfin makos are slightly larger/longer than shortfin makos, reaching a maximum length of about 4.3 meters. Makos tend to feed mostly on cephalopods (squid, octopus, cuttlefish, nautilus, etc.) and bony fish, but they are cannibalistic, and have been known to feed on small cetaceans (dolphins, porpoises, etc.) and chelonioideans (sea turtles).
The biggest things to remember about shark behavior, especially when they're hunting or eating, are the following. 1) Sharks neither snarl nor growl just before attacking, much less if they're not going to actually bite something. The supposed tiger shark that swims up to Carter in the beginning snarls and growls at him even though it doesn't actually bite him. First of all, sharks don't have vocal chords. They literally have no larynx (that's the organ that contains the vocal chords that produces vocal sounds). Secondly, sharks have no lungs, so growling would be impossible. They have gills in the place of lungs, but the gills have water running through them constantly to allow them to absorb oxygen from the water. That means that they're open to the water pressure around them. In other words, the water pressure in their gills is the same as the water pressure all around the shark. This makes growling impossible merely on that note. It's a physical impossibility, hands down. Thirdly, sharks bear their teeth when they're about to bite something. They're teeth are usually guarded from unnecessary damage by naturally pointing inwards, positioned inside their mouths. When they actually mean to bite something, sharks will pull up their "lips" so to speak and push forward, so to speak, their jaws and teeth somewhat out of their mouths so that they can carry out the motions involved in biting. This is not the same thing as snarling. Sharks absolutely DO NOT have an evil snarl, the way this film erroneously depicts. I know, they're supposed to be killer sharks out for human blood, but it's just not believable.
2) Many sharks, like the great white shark, my personal favorite of all the shark species, will take large bites out of medium to large prey, swim away as they swallow their chunk of meat whole, then swim back to take another bite. That's the way they eat. They don't just devour their pray whole in a crazy series of bites or one humongous bite. Just like great whites, makos will also preferably take smaller to medium-sized bites out of their prey, swallow those chunks whole, then quickly swim back for more. Great whites have large, triangular serrated teeth, especially on the lower jaw. When they bite, great whites will use their upper jaw to maintain a strong grip on their catch while they begin slicing into the flesh with their lower jaw. Unlike great whites, makos have much thinner, somewhat weaker teeth that oddly curve inwards, almost like fishhooks. The reason for this is that they will catch smaller prey than the larger shark species and swallow them whole. The inward-curving teeth help to ensure a tight, deadlock grip on their prey, even if it's a slippery cephalopod. The mako's teeth are also not serrated at all, but instead are smooth all throughout. While makos can and do take large bites out of their prey, they mostly hunt by biting hard on their prey, and if it's small enough, they'll swallow it whole. The sharks in this movie merely snarl/growl at whatever they're going to eat, charge in towards them at full sprint speed, and continually bite them over and over to devour them in seconds. One quick question: How and when are they swallowing the chunks of meat they're biting off? I'll tell you, while many people have negatively criticized "The Meg", the filmmakers of that movie at least took the time to depict realistic shark behavior, maybe not from the meg itself, but from the other sharks feeding off the whale and the fallen meg at the end of the movie. Even the meg in that movie adheres more to reality by not snarling or growling. It just looks at you with a blank expression. That's what a real shark does, no matter how smart or bloodthirsty it is. It's an animal, after all. Now, while sharks do engage in feeding frenzies, especially when the extreme scent of blood from a dead animal or person gets them excited, they will still not devour anything the way seen in this movie. Go watch "Open Water" (2003) for realistic shark behavior.
Okay, so now that I've covered the very basics of actual shark-feeding behavior, I'll movie on to the discrepancies this movie shows in depicting its various sharks in different situations. 1) The three enhanced "mako" sharks change appearance, shape, and size between their mechanical, radio-controlled, animatronic versions and their CGI versions. The CGI versions are always much larger and have a much rounder, less pointed snout. Carter states at the end of film that the last shark was 25 feet long. However, the director designed it to be 26 feet long, one foot longer than the shark in Jaws (1975). 2) The "tiger shark" that is shown in the beginning of the film isn't what a tiger shark looks like. Firstly, tiger sharks don't have such a pronouncedly-pointed snout. It's much more rounded off all throughout. It's much more stubby and very muscular. Secondly, tiger sharks don't actually have stripes running down their bodies. They have splotchy, spotted markings that run all along their bodies, especially from their gills all the way down to their tail fins, but they are not stripes.
On to a different type of discrepancy. All three enhanced sharks exhibit behaviors akin to crocodilians not to sharks, where they lie in wait below the surface of the water to explosively emerge and surprise their unsuspecting victims. The one that attacks "preacher" is unseen until it lunges toward the parrot. It then magically disappears before "preacher" falls into the water in slow motion while clinging to the metal kitchen shelf. Similarly, the shark that attacks Susan is "presumably" waiting for her in waist-deep water. First, it pushes the paper-weight shark model she had been studying on earlier towards her. She gets spooked and pushes it into the water only to realize that a real shark is just a few feet away, whereupon it thrashes and lunges toward her. The shark repeatedly lurks just beneath the surface of the water and lunges at her when she attempts to reach for her research data diskette. This is exactly what a crocodilian would do in a watering hole in the Savannah. A couple of quick questions: As she walks toward her private quarters, the floor is unflooded. That is, there's no water at all before she reaches her quarters. When she reaches her quarters, she literally unlocks and opens a pressure door and then walks down a few steps to finally enter the waist-deep water. She then actually closes the door behind her before proceeding to her locker. How did her chamber get partially flooded when the passageway leading to it is absolutely dry and the door was locked? More importantly, how did the shark get into her private quarters in the first place if the passageway leading to it is dry and if she had to unlock and open the door to it? Um... Clearly, there's absolutely no way that is possible.
A very similar "intentional" discrepancy or scientific impossibility is when the three enhanced sharks are snarling at Carter then swim backwards and away from him when he brandishes his tranquilizer gun. Sharks' fins, regardless of the species, all have fins shaped in a very specific, precise anatomical design that ONLY allows for forward motion when sharks sway their bodies through the water. In other words, as they sway their bodies from side to side, sharks will ONLY move forward because of the anatomical shapes their fins possess. Finally, the shark that grabs Jan, takes her underwater for a few seconds, then emerges briefly with her to permanently disappear also behaves like a crocodilian.
There's a deleted scene where Susan offers a speech of everyone's work, congratulating them and thanking them for their dedication. Everyone is in a festive mood. The ambience set by the music as well as by Susan's words is greatly emotive of her humanity and heroism. That scene alone would've set the tone of the entire movie to being very personable, to having a much more satisfying emotional character arc for Susan. It would've been much more satisfying that having the movie abruptly change from being a pseudo-scientific movie to being a shark slasher film.
Most, if not all of the dialogue, in this film doesn't make much sense or does anything to advance the overall plot of the movie. When everyone is aggressively questioning and snapping back at Susan (especially Carter and Franklin) about what she did do the three enhanced sharks, Carter very strongly implies that Susan used everyone, especially him. Carter makes it a stinging point that that's what the problem with her methods is, that she used everyone to reach her goals. Later on, when both Susan and Carter are in the bathroom in Jan's private quarters where Carter is applying a type of viscous, presumably "oily" substance to the cut on his arm (He says that he's coating it with oil. It doesn't look like oil, though.), Susan tells him, "Carter, what you said down there... maybe you were right. Everything I've done... I don't think I could've done it without you." Carter replies, "I know, Susan. That's the problem." Wait just one minute! Carter had just twenty minutes earlier blasted her for apparently using him and everyone else who worked there, even Brenda, who was in the observation tower. Everyone had apparently agreed, with Janice calling her a derogatory name. They had established that her using everyone and unethically genetically manipulating the sharks' DNA were the quintessential problem. How in the world does Carter now say that her depending on him to do everything is the problem?! That makes absolutely no sense! There are countless examples of such dialogue, where people's words don't flow at all with the plot. There appears to be no reason, no logical excuse, for people to say what they say. In that same scene in Jan's private quarters, when Carter tosses a zip-lock bag containing several energy bars at "preacher", he states, "Even in death, standards." What in the world does that refer to or even mean. Carter is simply giving them the energy bars so that they could all keep their energy up as they have to continue treading through water, which is extremely energy-consuming. Yes, it's true that Jan just died and that they're in what used to be her quarters and that they'll now be eating what used to be her energy bars, but that statement doesn't correlate with having to keep up your strength to get out of there.
Another example of this is toward the beginning right after Carter talks with Susan (The end of that discussion also has Carter getting angry for no reason when Susan tells him that if he gets fired when they all lose their jobs for not meeting their deadline, he won't be able to find work. I believe that she's right since he's an ex-con.), Carter asks Scoggs if he raised "the fences, like I asked". Scoggs says, "I took care of it". First of all, the raising of the perimeter fencing would possibly require hundreds of thousands of dollars and would definitely require many people with machinery and boats to install extra portions of fencing. That sounds like it would require a corporate decision and would most likely take several days to several weeks to complete once the decision would be approved. Yet, in this movie, Scoggs was able to pull it off by himself in less than a day. Yeah, that's not possible at all. On this same note, titanium isn't indestructible, as is implied in this movie. It's quite a bit stronger than steel in many respects, but it will still bend and eventually fail.
Let's talk about when Carter uses his specialized, superman/jedi swimming abilities. Firstly, you NEVER, EVER turn your back to any wild animal. Sharks are no exception. You must always remain alert and ready to act, but not nervous, which is much easier said than done. That is, however, what every documentary will show you from divers who go into the water either in a shark cage or with no barrier between them and the sharks. You must always keep the sharks swimming around you within your view. Sharks can actually see very well, especially when the water isn't too deep where there's a lot of light shining through. They'll know that you're looking at them and, while they might be curious and come close to you, they'll more than likely not want to engage in activity with you, so they'll turn and swim away. Showing your back to any predator in the water, especially sharks, will tell them that you're easy prey. It will make you vulnerable to attack, and they might just lunge at you to see if you're worth eating.
Carter jumps into the water with the tiger shark and just swims away while the shark approaches. It bears its teeth and growls just before Carter turns around and sort of does a weird ride-'em maneuver akin to that of killer whales and their trainers. That's probably what the director, Renny Harlin, wanted to show, and he probably also wanted to show just how amiable regular sharks can be compared to the three enhanced ones. I already talked about not showing your back to sharks, but we must discuss the nature of shark skin. Shark skin is not smooth, like that of orcas or cetaceans. It's actually extremely rough, similar to the roughest sandpaper you can buy. Carter "rides" the tiger shark without a shirt and in shorts. This activity would've certainly caused severe peeling of his skin in any exposed areas. It might even cause severe bleeding. Similarly, when Carter has tranquilized one of the enhanced sharks and taken it into the facility for extraction of the protein complex, he keeps patting the shark and running his bear, ungloved hands along the shark's back. Yeah, again, this would cause serious injury to his hands.
Going back to when Carter tranquilizes one of the enhanced sharks for the protein complex extraction, as soon as he shoots it, the tranquilizer immediately begins working in the shark's circulatory system, which means that the shark will almost immediately begin to sink as it quickly becomes immobilized. How did Carter get the shark onto the loading platform that is seen rising with the shark into the medical facility? There was no such visible platform in the scene where he's shooting it. It's assumed that he shoots it in open water, meaning that there's nothing around for many miles, just the deep. Now, assuming that Renny Harlin tried to make his "mako" sharks as much like the shark in Jaws as possible, then a shark of that size would easily weigh anywhere from one to two tons (that's 2,000 - 4,000 lbs), even though Franklin and Carter tell the audience that they weigh 8,000 lbs, which would be 4 tons. To put things in perspective, that's heavier than a regular pick-up truck. A pick-up truck can weigh about 2 tons, but never 4 tons, at least not on its own.
Okay, let's cover some of the gibberish psuedo-science in this movie. A little backstory, according to what Susan says: The protein complex is supposed to reactivate inactive Alzheimer's patients' brain cells, which means that it's supposed to enable them to run and process synapses properly. However, it means much more than that. The term "inactive" alludes to the fact that the brain cells are literally dead. So, Susan and her team are hoping that the protein complex will literally bring life back to those dead brain cells. Harlin probably thought this was clever and was aiming at trying to make Susan seem like a type of Dr. Frankenstein, which is most likely the reason that he eventually decided to kill her off. The procedure in the movie merely has Susan drenching a sample of "cultured inactive brain neurons of an Alzheimer's patient" in "2 cc's of the protein complex" (cc's would be cubic centimeters, a measurement of volume). So, there are several things wrong with the proposed science here. 1) Susan mentions they'll be testing the protein complex's viability on a sample of what she calls "cultured inactive brain neurons of an Alzheimer's patient". To begin with, whenever something is cultured, it has literally been cloned and grown within test tubes and the such. If they're inactive, they won't grow because they're literally dead. They'll just rot. Secondly, it is impossible to clone and grow, to literally culture, brain cells. It simply cannot be done. 2) As Susan watches the screen of the interaction between the protein complex and the brain cells, she says the following: "Now, what you're looking for here is lightning in a bottle. Protein complex is interacting with the neurons. Neurons are becoming hyperosmotic. Membrane integrity's improving. They're firing! They're firing!" Firstly, in order for the membrane integrity to improve, there would have to be long process or growth and repair in the neurons, which is highly unlikely at that level and certainly wouldn't take a few seconds. Secondly, the neurons wouldn't transmit electrical signals the way the movie shows them emitting lightning. The signals would travel in the direction that the neural pathways are formed, not out into space to an adjacent neuron. Thirdly, the "lightning" shown here would not be possible since brain cells are drenched in a fluid of some kind. Fourthly and fifthly, simultaneously, the captions for one of Susan's remarks are given as, "Neurons are becoming hyperosmotic", but she actually verbally says, "Neurons are becoming hyperautomotic", which is not a real word (automotic is not a real word). The word "hyperosmotic" refers to osmosis, which is the continual, spontaneous back-and-forth transfer of solvent molecules through a semipermeable membrane from an area of less solute concentration to one of higher solute concentration, which eventually leads to equilibrium in the solutions' concentrations on either side of the membrane. The proteins are too large for this.