Now - there are many flaws with the film. There are moments that it is all so ludicrous it makes it hard to suspend your disbelief through them, and this is not actually how most sharks are (particularly they wouldn't necessarily be in shallow water - or so I'm told). But overall those moments are few and far between.
Lively really holds her own as the solo star of the film, you can feel her desperation and she truly had me worried for her. I mean, she is a national treasure we can't lose her this young.
Most impressive was the use of music and sound, as well as the cinematography. The sound quieting under the water, paired the overhead and underwater shots all contributed to an overall beautiful movie to watch. And even though there's the threat of dying on a rock or in a shark, it made me want to go there and surf (but with lots of other people).
Truly though, it was a frightening movie. The tension was so great that I was personally hiding my eyes most of the movie, and there were multiple times that most people in the audience jumped and screamed.
If you're looking to be scared, this will definitely do the trick.