He ordered the crab. She was tempted by the tuna. We shared an appetizer of shrimp by a swank hotel pool. How appropriate. Are we not all, on some cosmic level, bait?

Which brings the husband- and-wife team of filmmakers Chris Kentis and Laura Lau, creators of the indie thriller "Open Water," to the question of sharks, the ultimate evolutionary creation of dietary simplicity. The original South Beach diet: all protein, all the time.

Kentis and Lau agree. On land, snakes freak us out. But in the deep blue sea, the shark has so imprinted itself upon our subconscious that anyone who ever swims in the ocean always possesses, in the back of the mind, the dark fear of the big bump from below.

And sharks -- gray reef sharks and bull sharks -- averaging seven to 12 feet and filmed in the aquamarine waters of the sunny Bahamas, are the Freddy Kruegers of "Open Water," which opens Friday in Washington.

This film is no high-tech marvel of computer-generated creatures. All the fish in "Open Water" are real, though the production notes for the movie remind us these Bahamian sharks have "had lots of exposure to people." (Is that good or bad?)

You know how Kentis and Lau made their shoestring movie, about a yuppie married couple accidentally left behind in the sea on a tourist diving trip? With bloody slabs of meat. Watched carefully by professional shark wranglers, the actors, Daniel Travis and Blanchard Ryan, bobbed like chum. "When they wanted the sharks to swim really close, they would throw the chunks of tuna right next to us," Travis says.

Neither actor was munched, but Ryan was nipped by a barracuda on the first day of filming.

The movie generated buzz at the Sundance Film Festival in January, was bought by distributor Lions Gate Films, and early reviews have been positive, declaring "Open Water" an intimate "Jaws," a grass-roots thriller, a micro-budget nail-biter.

Kentis understands, of course, that this film lives in the wide shadow of "Jaws," but he and Lau found their inspiration in a little news item in a dive magazine (they're both certified divers who live in New York) about a couple who were left behind on a dive trip to fend for themselves. Kentis says he was also gripped by the story (memorably told by the Robert Shaw character in "Jaws") of the USS Indianapolis, the World War II heavy cruiser torpedoed in the Pacific, whose crew spent five days in the water. Nine hundred sailors and Marines went into the drink, 317 survived, the dead taken by thirst, drowning and sharks.

The question of the film, and the one audience members will ask themselves, is how you would act floating upon a shark-infested sea with your spouse? "It is a very elemental movie, on some levels, and it asks, 'What are your thoughts?' " says Lau. "We wanted the audience to put themselves in that position, for them to become the two actors."

In addition to the sharks, the sea and sky -- in all their tropical splendor and darkness -- share equal billing in "Open Water," which was shot on digital video by Kentris and Lau using their life savings of $120,000, with family members serving as crew, mixing fake vomit and holding the sound booms. It is a reminder of the fine filmmaking that can be done on a tight budget by two people with a vision.

The film is naturalistic, and therein lies its terror. In most shark movies, the creatures from the cinematic deep are evil. But in "Open Water," they act like real sharks. They do not attack -- they probe, they bump, they seek to know: Are these floating humans bleeding yet? And if so, can we eat them now? It is beyond morality; it is recycling. Which does not make it any less scary for anyone who enjoys the water. Kentis and Lau stress that it was not their intent to demonize the sharks, who are only performing their natural role. It is, rather, to glimpse amid the rising and falling swells what would we do in those hours, husband and wife, floating together -- but also so alone.

Chris Kentis and Laura Lau, above, made "Open Water" on a shoestring budget, but got more than their money's worth from actors Blanchard Ryan and Daniel Travis as divers accidentally left behind by their tour boat.