Surfing on a secluded Mexican beach, an American medical student (Blake Lively) is attacked by a shark. Wounded, her blood staining the water, stranded on a tiny rock with the tide coming in, she struggles to survive and outwit the shark, who circles her relentlessly waiting for one false move. It’s Jaws meets Gravity, with the attacks (which are quick and brutal, disposing of three hapless extras while Lively watches in horror) taking a back seat to a story of survival, as Lively’s character calls upon her medical education and personal resources she never knew she had to stay alive from hour to hour. It’s a one-woman show for Lively just as Gravity was for Sandra Bullock, well-written by Anthony Jaswinski and showcased by Jaume Collet-Serra’s tense direction and the expert editing of Joel Negron. J.L.