Not unlike a grim round of the game “Would You Rather?” — in which the choices are getting eaten by a shark or running out of oxygen underwater — “47 Meters Down” stars Mandy Moore and Claire Holt (“The Originals”) as sisters facing not one but two potential ways to die.

And while it’s no “Jaws,” or even “The Shallows,” “47 Meters Down” is a lean and mean little B-movie that skillfully plays on our fear of what lies beneath the waves. Coming into this thriller with a fairly unimpressive horror résumé, director Johannes Roberts (“The Other Side of the Door”) ratchets up the tension with precision, making sure that we’re always aware of the presence of the sharks and the impending absence of air in the scuba tanks.

Kate (Holt) joins her sister Lisa (Moore) on a Mexican vacation, but Lisa eventually breaks down and admits that the reason her boyfriend didn’t come instead was because he dumped her for being too dull. It’s understandable, then, that after a night out where the ladies meet two cute locals, that Lisa can talk the normally risk-averse Kate into scuba-diving in a cage to interact with the local shark population.

Watch Video: Mandy Moore Fends Off Sharks in '47 Meters Down' Trailer

Captain Taylor (Matthew Modine) is an amiable enough seaman, but Lisa is right to balk at the rickety-looking winch and cage in question. The device works when it sends the guys down, but for the girls, the chain snaps, sending Kate and Lisa plummeting down the titular 47 meters to the ocean floor. They can’t just swim up, first because of the sharks circling overhead, and later because they’ll get the bends if they don’t spend five minutes at a median level before surfacing. (And that five minutes would have to be spent amidst the aforementioned sharks.)

The characterization in the script by Roberts and Ernest Riera is fairly minimal, but it’s enough to create the situation and the perils therein. (Kate is a trained scuba diver, while Lisa is on her very first plunge.) There’s no B-story here, either; once the women are stuck in their situation, it’s almost entirely on them to figure out how to survive. (Captain Taylor is on the radio, but they can only get his signal when one of them swims closer to the surface.) But even for a tale that’s this linear, the writers manage to work in a surprise or two.

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The Dominican Republic stands in for Mexico, with water tanks in Britain subbing for the Atlantic, but “47 Meters Down” never feels stagebound; when Lisa swims out to get a flashlight, and then finds herself lost in the dark, unsure how to get back to the cage, the ocean feels terrifyingly immense and infinite.

Moore and Holt both give fine performances, particularly with the impediment of wearing oxygen masks for most of the movie, and they convey their degrees of panic and desperation effectively, even in a cloud of air bubbles.

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This is a B-movie in the best sense, and it harkens back to an era when major studio releases weren’t just B-movies with stars and a budget. It’s lean and mean, focused and direct, and the jolts are both effective and well earned. Roberts and editor Martin Brinkler keep their ticking clock tightly wound, deviously making our heroines’ plight seemingly more hopeless with each passing moment.

“47 Meters Down” isn’t great cinema, but it’s exceedingly effective moviemaking that gives PG-13 horror a good name.