Just when you thought it was safe to go back in the water, the movie shark rears his ugly fin once more. Make that twice more: In last year’s “The Shallows,” Blake Lively played a young surfer stalked by a great white, and in the new release “47 Meters Down,” Mandy Moore and Claire Holt play sisters trapped in a shark cage at the bottom of the Pacific Ocean. The snaggletoothed villain hasn’t changed much since he prowled the coast of New England in “Jaws” — he’s still sneaky, smart and unnaturally attracted to human flesh — although in these new films he has apparently migrated to Mexico.

That’s where the heroines of “47 Meters Down,” sunny blonde Kate (Holt) and fretful brunette Lisa (Moore), are on vacation. Lisa is going through a devastating breakup, which explains why she throws caution to the wind and joins her sister for a dive in a shark cage. The boat captain, a scruffy expat named Taylor (Matthew Modine), seems like a decent sort, although his aging equipment is another matter. The cable suspending the cage is awfully thin and rusty — a bit like the movie’s premise — but Lisa takes the plunge anyway. Needless to say, she’ll regret it.

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“47 Meters Down” is basically a survivalist dilemma: Stay put with a dwindling oxygen supply, or scramble through shark-infested waters to the surface? It’s also a filmmaker’s dilemma: How to keep us watching a dimly lit underwater movie whose lead actresses are obscured by scuba masks? Director and co-writer Johannes Roberts (last year’s horror flick “The Other Side of the Door”) does fairly well on all counts given his self-imposed limitations. He employs extreme close-ups to create tension (and help us distinguish between the two sisters), and he uses the blind murk of the ocean to keep us wondering what might jump out of it.

In case you’re wondering, most of the dialogue is spoken through underwater radio gear — not that anyone says much worth hearing. If you’d just escaped the jaws of a great white, wouldn’t you scream something more colorful than “The shark almost got me”? All told, “47 Meters Down” creates about as much tension as it can before it finally snaps and drifts away.