It could be Jaws with a Blair Witch sensibility, but instead Chris Kentis' low-budget thriller Open Water goes adrift about halfway from shore. He speculates about the intriguing real-life tale of a young couple left to the mercy of sharks on a scuba-diving trip, but he sinks under the weight of trying to pack those lost hours when the fanged fishies were circling their prey. Like so much live shark bait, this story winds up missing the vital middle parts.

Unknowns Daniel Travis and Blanchard Ryan play yuppie couple Daniel and Susan, so bogged down by work they have to book time to see each other. They hope a tropical vacation will soothe their tense relationship but instead find themselves in deep water, literally.

"TREADING WATER"

They endure the most gruelling hours of their lives - particularly the night hours when a lightning storm sweeps them further into a murky unknown. In fact, the night-time scenes are the most cinematic - a bleak and menacing situation made more chilling by the strobe effect of lightning over black water. But the sequence is too short, and once the sun rises, so too does the feeling of watching a made-for-TV reconstruction.

Without special effects and animatronic beasties, Kentis reverts to the power of suggestion to generate tension. Annoyingly, the manic plucking of violin strings always builds to nothing, and when you do finally catch a glimpse of the sharks, they look so piteously undernourished, you don't begrudge them the meal.

As they bob around in the quieter moments, the underlying resentments of the twosome slowly bubble to the surface and provide some wittily observed kitchen-sink drama. But again, it all too quickly slips down the plughole. Kentis ends up treading water until, finally, the whole film gets sucked under.