Jaume Collet-Serra (Unknown, Non-Stop) directed this nail-biter about a Texas surfer and medical school dropout (Blake Lively) who’s terrorized by a great white shark in the waters off a secluded Mexican beach. Lively’s athleticism more than compensates for her limited emotional expressiveness as her character tries to outsmart the relentless predator, a preternaturally malevolent specimen that can chomp through a steel buoy. Her companion throughout this long, lonely ordeal is a wounded seagull, the movie’s version of Wilson the volleyball in Cast Away. This is silly stuff but often genuinely scary, with excellent aerial and undersea photography and some persuasive special effects. You want realism, watch the Discovery Channel; this is pure escapism.