I wrote this entire review and then my laptop freaked out and deleted the whole thing. I really don't feel like writing it all again, so...
Blake Lively is talented and fierce! The shark isn't scary because the movie surrounding it is too predictable! Design choices that tell you exactly when the shark will strike remove too much tension!
The tone is fickle: at times the movie embraces its b movie silliness (bird sidekick! Shark with a grudge!), at times it takes itself too seriously (Blake Lively's family subplot- just let her be Blake Lively fighting a shark!), other times it gets too unrealistic (the entire final battle!).
And the director uses slow motion far too damn much- like he thinks it's some higher art form.
Too melodramatic to be a great b movie, too guiltily pleasurable to be a great thriller. The Shallows lands somewhere in between: trim a little fat and speed it up a bit, and it could be a successful b movie.
Just let a girl be a girl and a shark be a shark, you know?