The setting is lovely but the effects chintzy in Jaume Collet-Serra’s “The Shallows,” a mash-up of “Jaws” and “Open Water” that might have been written on a very small napkin (though scripter Anthony Jaswinski uses any device he can muster, including lots of talking-to-oneself by the main character, to expand the dialogue). It’s also burdened with a big finish that comes off as totally ludicrous—and not simply because of those poor shark effects.

Nancy (Blake Lively) is a medical student who comes to a beautiful but remote Mexican beach to surf in honor of her recently-deceased mother, who once visited the place. Unfortunately, she’s attacked by a great white shark that’s invaded the coastal waters and leaves a big gash on her leg. The beast is more aggressive than Spielberg’s Bruce (or even Bruce’s mother, who apparently showed up to avenge her offspring in one sequel by singling out particular people for gobbling). It’s already killed a whale, leaving the bloodied corpse for gulls to feed on, and now isolates Nancy first on a rock, and then on a nearby buoy. While she’s trapped, the shark takes the opportunity to chow down on a drunkard who wanders onto the beach and tries to steal Nancy’s belongings (including her bobbing surf board), as well as two local surfers whom she’d encountered when a likable fellow named Carlos (Oscar Jaenada) initially dropped her off. The keys to Nancy’s possible survival are not just her own ingenuity and grit, but a helmet camera worn by one of those doomed surfers, which—as a prologue shows us—is discovered by a kid on shore.

One has to give credit to Lively for endurance (and an attractive physique, which her bikini shows off to best advantage), if not for a stellar performance—her reading of the lines she recites to tell the audience what’s happening when the images can’t is pretty flat. (In fact, she’s outacted by an injured seagull that she shares the rock with—this movie’s equivalent of Tom Hanks’s volleyball Wilson in “Cast Away.”) In what’s effectively a one-woman, one-shark tale (not counting the comic-relief seagull, of course), she carries things as best she can, although Jaswinski’s effort to provide Nancy with a background for us to empathize with via skype conversations with her father (Brett Cullen) and sister (Sedona Legge) falls miserably flat.

Cinematographer Flavio Labiano’s widescreen images capture some lovely views of the cove—a few of them breathtaking shots from far above—but when he’s charged with following the waterlogged action, he and editor Joel Negron fail the clarity test, though admittedly they’re hobbled by poor process shots that might have come from a Shark Week marathon on the SyFy Network. Marco Beltrami’s score works hard to ratchet up the tension in the big moments, but the means he employs are fairly generic.

Occasionally minimalist thrillers like this one can make a reputation: when Spielberg helmed “Duel” for ABC, for example, that cat-and-mouse exercise was both clever and imaginatively executed on a shoestring, and helped to launch his career. Unhappily Collet-Serra’s attempt to follow in his illustrious predecessor’s footsteps falls very short. When “Jaws” came out in 1975, one of the advertising taglines was “Don’t go in the water.” For “The Shallows” one can suggest an alternative: “Don’t go into the theatre.”