Dir: Chris Kentis. US. 2004. 79mins

Based on a true story about a husband and wife who were accidentally left behind while scuba-diving 18 miles from shore, Open Water, which played in the American Spectrum sidebar at Sundance, is a bargain-basement scary movie that should more than repay its investment. What it lacks in cinematic terms - a long list indeed - is more than made up for by the terror it will strike in the hearts of filmgoers looking for a serious jolt. It's easy to see why Lions Gate picked the film up out of Sundance. Viewers will be lured by a trailer that focuses on the popular fascination with sharks, the terror of the characters and the nasty sharp teeth of their tormentors.

Stressed-out urban professionals Susan (Ryan) and Daniel (Travis) have decided to take a Caribbean holiday. He packs up the SUV and waits while she handles the last of her phone calls. They go to the airport, they get on the plane, they get to their destination, they shop in the local market, they climb into bed, they make a dispirited attempt at sex, they get up, they go to the dive boat.

Unlike a standard genre title, this film never hints at what is in store, establishing a normalcy that will cast in sharper relief the coming nightmare. But Kentis has overdone the normalcy part: through the first 40 minutes, the film verges on tedium. There's no character development, no back story to speak of. The set-up of how they come to be left behind in the ocean is poorly handled - the captain heads back to shore with two of his air tanks unaccounted for - and the floating couple's dialogue seems oddly calm given the severity of their situation.

Festival wags have referred to the film as Gerry On Water, a reference to Gus Van Sant's slow-paced, desert-based drama. Unlike that film, the victims' faces don't reflect the toll that would come from hours and hours of exposure without water. But when the nightmare starts, the film begins to grip and grip to an excruciating crescendo.

The concept of Open Water promises more than the film delivers - there are so many ways it could be better - that it cries out to be remade. Most puzzling, given the presence of a search-and-rescue helicopter, the film doesn't present the vast space through which the abandoned couple is drifting.

Lions Gate will likely smooth some rough edges before allowing the film to float freely in the cruel commercial seas. As is, it's hard to imagine strong word of mouth, especially with an unnecessary downer ending. While audiences are more accepting of down-beat endings when the nemesis is supernatural, as in The Blair Witch Project, a real-life personal disaster - be it on a mountaintop as in Touching The Void or a shark attack - is something we all hope to survive.

Production co: Plunge Pictures
US distribution: Lions Gate Releasing
International sales: Lions Gate
Producer: Laura Lau
Screenplay: Chris Kentis
Cinematography: Chris Kentis, Laura Lau
Editor: Chris Kentis
Main cast: Blanchard Ryan, Daniel Travis, Saul Stein, Estelle Lau