This is one of those movies where 'it looks great' to the layperson, but the tech and 'backstory' is so bad, that if you know ANYTHING about diving, you realize this started 'jumping the shark' (pun intended) from the first few minutes.
From poorly explained dive science (you don't 'automatically' get the bends from nitrogen bubbles 'on the brain') and you don't put a group of tourists in a shark cage without a knowledgeable guide (who is an experienced diver). You don't have sharks hunt you like a meth'd out manhunter, either...and when you dive without a wrist compass, or dive knife...well...idiot tourists SHOULD be on the 'menu'. This was a movie MADE-BY-MURPHY about just how much POSSIBLY COULD go wrong on a number of dives...not all in one dive!
I was left with questions like 'what is the drama here, that they dove with sharks for their first time, that the sharks wanted to eat them, that they were going to drown from lack of sufficient air, or that they were locked in a cage at the very lower limits of 'amateur diving'? I was confused...and about mid-movie, I realized...the shark was so much more on his game than the 'shady charter boat captain', his 'non-experienced shark assistants' or the moronic girl clients that they put in a shark cage without a guide....so, I was FORCED TO ROOT FOR THE METH'D OUT MONSTER KILLER SHARKS (which were Great White Sharks...called 'nurse sharks' once in the script, just for 'OMG' good measure).
AVOID THIS MOVIE...GO SEE SHARKNADO...at least it doesn't take itself THAT SERIOUSLY!!!