Good tension combination using confined spaces, risk of drowning, plus the immediate threat of sharks. The acting was acceptable. The actual shark models used looked great. Decent use of camera panning and perspective changes to augment the escalating moods throughout the film.
Unfortunately, nearly every moment of suspense was far too predictable that it detracted from the horror of each situation. The audio and music crescendos intended to boost suspense and divert expectation, ended up giving each death away, as it's done in the same way as nearly every modern horror film. Situations like:
"Hmm, I haven't heard any music for a few seconds during this monologue. It's as if character X is about to be killed in 3...2...1...aaaand yep, there they go..."
Other than clarifying the protagonist's geeky personality at the beginning of the film, the purpose of the bully seemed rather irrelevant to the movie, especially with the last scene she is in. Overall, the protagonist being a geek really didn't add anything to the film as well.