“Under Paris,” “Deep Water,” “Thrash,” “Chum” — the shark movies come so thick and fast these days, there’s scarcely any time between them to deem it safe to go back in the water. That said, if this recent run of films is to be believed, regular old beaches are fine these days. Cue an initial sea-creature attack less bloody than those to come, but perhaps the film’s most eerily and alarmingly constructed setpiece. (The safer cave route he recommends is marked on the map in green; the one they choose is marked in red. A more immediate priority is navigating a route out of this cave network, with the shark, given limited room to roam, tailing them at every turn.