Some frustrating things
I thought the scene where Matt was taken by the shark was a little weird. Luke was wearing goggles and checking under the water constantly to see what might be following them beforehand, but as soon as Matt was pulled underwater by the shark, it was like Luke forgot he could just look underwater to try and find him. Instead the three of them are huddled in a circle, panicking.
Also, it bugged me whenever the shark would make an appearance, the group would begin thrashing and splashing about, creating bubbles, which everyone knows attracts the sharks. Even Luke, who made an effort to keep everyone from splashing in the start of the film, spends an awful lot of time splashing around himself.
Other than that, the movie felt very realistic. I've never found myself in a situation like that, so I can't really vouch for much, but it had me wondering what would I do and stuff like that. I think the realistic factor is what causes minor unbelievable things in films like these to be more noticeable. A great thriller anyhow!