Movies that set out to frighten us almost always have a structure that is secretly reassuring. The audience knows, more or less, when the scary climaxes are coming, and we let out a collective sigh of relief when they're over. Open Water, the terrifying new independent thriller, doesn't let us off the hook in that way. Chris Kentis, who wrote, directed, and edited the film, and also served (along with his wife, Laura Lau) as its co-cinematographer, shot ''Open Water'' on digital video, and he uses the slightly flat, present-tense quality of DV to create the illusion that everything he shows us has been caught on the fly -- that we're seeing a drama in the style of a documentary about an all-too-plausible nightmare. In a movie like this one, style becomes suspense, with the zero-frills video aesthetic telling us, in every frame, that there is no cozy protection, no higher power -- not God, not a studio focus group -- to sway and redeem the outcome.
The characters are on their own, and so is the audience. For most of ''Open Water,'' we're staring at nothing more than the sky, the endless ocean, and a pair of nerve-jangled scuba divers, Daniel (Daniel Travis) and Susan (Blanchard Ryan), who are stranded in the middle of it. The two have been left behind, accidentally, by a resort boat during their vacation on an unspecified Caribbean island. As they float and bob and argue in the water, trying to figure out how to save themselves, not much time goes by before they catch their first glimpse of a shark fin. The two could be attacked at any second -- or not. When one of them feels, say, a sting on the leg, we suck in our breath: Is this the moment we're going to have to watch someone's limb bitten off?
In spirit, ''Open Water'' reduces us to children peering through our fingers, waiting for the horrid deliverance we're not quite sure we want to see. The film is based on real events, but it also ratchets up the jittery off-beat rhythms and harrowingly unstable, surface-of-the-water eye views of ''Jaws''; if anything, it may be even more intense in its gathering ripples of dread. Kentis is that rare director, a humanist who is also a teasing sadist.
From the start, he draws us with voyeuristic cunning into the tensions between Susan, a no-nonsense beauty with a high-stress job as a TV sports producer, and Daniel, the geeky ''enlightened'' mate she needles in order to work off her resentment. They're presented as an archetypal 21st-century couple. She's the stronger of the two, which, ironically, is the source of her testiness: She's with a man who's too namby-pamby in his empathy. Their first night of vacation, he fails to arouse her sexually, and Kentis, who draws lifelike performances from both actors (Ryan, who resembles a blowsier Charlize Theron, is especially good), does a marvelous job of letting this routine domestic discord slide into a generalized mood of unease. On the boat the next morning, the camera is almost cruel in its mingling of objectivity and silent fate, observing a random series of events -- a forgotten diving mask, a mistaken head count -- that will result in the boat leaving to return to shore without them.
The couple's gradual realization that they've been forgotten, abandoned without a thought, is a powerfully disquieting reminder of how the protections of civilization can fall away. ''Open Water'' becomes a slow and steady descent into pure fear. Kentis has a born filmmaker's instinct for physical detail. He uses the sound of lapping waves to submerge us along with Susan and Daniel, and he turns the simplest of events -- a plane passing overhead, a desperately brandished knife -- into gripping motifs of hope, vulnerability, and danger. He also exploits the situation for a kind of nervous yuppie gallows humor. ''We wanted an ocean view,'' screams Daniel. ''Boy, did we get it!''
Open Water has a knowing edge. It counts, with a wink, on our awareness of ''Jaws,'' with its merciless vision of what a shark can do, and also on the lore that Steven Spielberg's film helped usher into popular culture -- the notion, for example, that a shark's instinct is generally to swim past humans in the water, but that they're drawn like addicts to the spill of blood. The sharks that we see on screen here are all real, and that ingenious use of the ultimate no-budget special effect cuts both ways. The smooth gray bodies, with their flattened heads and grimaces of teeth, make you queasy with anxiety -- you can't help but meditate on what that awful bite might feel like -- yet if the actors themselves weren't actually in mortal danger, perhaps the characters aren't either. Right to the end, you have no idea what's going to happen in Open Water. When that end arrives, it's a singular and haunting moment: an image of terror acknowledged and transcended. Somewhere, I believe, Alfred Hitchcock is smiling.