Stick two hot siblings (Mandy Moore, Claire Holt) vacationing in Mexico in an iron cage, dunk them in the shark-infested deep, and hope the Dolby Atmos and visual effects work will do most of the heavy lifting. One anticipates the random acts of dopiness often associated with this “if it can go wrong, it will” school of storytelling. But the cooked-up moments of imbecility in writer-director Johannes Roberts’s handling of the explicative exchanges between sisters are so oafishly handled they make sections of Sharknado look like documentary realism. Moore’s singular motivation: her boyfriend dumped her on the grounds of being boring and she’s going to save the day if for no other reason than to line her Facebook wall with vacation snaps to make him jealous. Not as bad as originally feared, thanks in large part to Mark Silk’s silken underwater cinematography and a sparingly effective use of menacing CG marine fish. (2017) — Scott Marks