If you happen to be at your summer beach rental and you are reading this review before a morning swim, stop now and wrap a rockfish with it instead. Wad the paper up to start the grill later. Use it to clean the living room window. Whatever you do, don't read it -- or, worse, go see "Open Water" -- until you're miles away from the ocean, preferably sometime next January. Even then, make sure you're someplace where you can safely draw your knees up to your chin and utter the occasional squeal of sheer dread.

"Open Water," a taut, riveting, uncommonly well-made film about two divers stranded for more than 24 hours in the middle of the ocean, is the type of movie that commands respect even from viewers who don't get their thrills from being scared out of their wits. Those adrenaline addicts will be more than well-served by the movie, whose nerve-jangling suspense and low-tech realism outpace even "Jaws" for all-out cinematic terror at sea. But filmgoers who aren't necessarily fans of this genre but who admire storytelling at its most pared-down and primal will no doubt also give "Open Water" an approving nod -- that is, if they can take their hands away from their eyes long enough.

Susan and Daniel are two tanned, toned young professionals who are so high-powered, so in the loop, that Daniel calls Susan on his cell phone to confirm her arrival time -- from the house to the car he's waiting in outside. They're stressed out, they need a vacation and they're taking one, to an unnamed island surrounded by turquoise water and scenic coral reefs. Once there, they sign up to go scuba diving, a junket they're well prepared for, both perfect physical specimens having earned their deep-sea diving certifications.

Disdaining their peers on the "cattle boat," they immediately peel off to do their own thing, a move that, combined with a confused head count later on board, results in their being left behind. (Reportedly, the movie is based on similar real-life stories.) "Open Water" chronicles the couple's fight to stay afloat and alive in an ocean teeming with creatures that see them not as Masters of the Universe or even garden-variety predators, but as one thing, pure and simple: food.

The horrifying indifference of the food chain is what filmmaker Chris Kentis captures so adroitly in "Open Water," and it's what gives this film a power far more terrifying and haunting than that of the anthropomorphic projections of Steven Spielberg. The jellyfish, barracuda and, yes, sharks that bump up against and occasionally bite Daniel and Susan aren't the weirdly vengeful cousins of Bruce the Shark; rather, they're the state of nature incarnate -- indiscriminate, voraciously self-preserving and, ultimately, frighteningly amoral.

Kentis and his wife and filmmaking partner, Laura Lau, shot "Open Water" on location in the Bahamas, where bull and gray reef sharks -- some more than 10 feet long -- are used to human swimmers. There, for 120 hours, newcomers Blanchard Ryan (Susan) and Daniel Travis (Daniel) swam and treaded water while the filmmakers threw raw meat in the water to encourage the sharks to swarm.

Using such old-fashioned tools as superior writing and urgent, pulse-quickening edits -- and without a single computerized effect, camera trick or scene of gratuitous gore -- the filmmakers plunge the audience directly into the couple's plight, as the characters first argue about who got them into this mess ("I wanted to go skiing!" Susan screams during one of several unexpectedly funny passages), then try to reason their way out and finally engage in the simple struggle to stay alive. Along the way, the ocean becomes a dynamic third character of maddeningly quixotic moods. Flat and glassy one minute, roiling the next, the sea -- as captured by Kentis and Lau's outstanding cinematography -- becomes an all-encompassing force of mesmerizing and often disorienting beauty.

As yuppies hoist by their own petard of arrogant exceptionalism, Ryan and Travis make promising screen debuts; Ryan is particularly stunning, resembling a younger Sharon Stone with her dazzling smile and hint of interior steel. While the immediate concerns of "Open Water" are the most basically human, on another level it's about a relationship and whether it will survive. Ryan and Travis tell both stories so well that, by the film's moving final scenes, the audience has actually come to care about their sometimes irritating but essentially vulnerable characters. Rarely have the dangers of drifting apart been given such a visceral and genuinely upsetting emotional wallop.

Open Water (79 minutes, at area theaters) is rated R for language and some nudity.

Blanchard Ryan and Daniel Travis as a power couple stranded among creatures with their own agenda.Daniel Travis and Blanchard Ryan try to have a fun vacation, but instead must face the hazards of "Open Water."