JUST when you thought it was safe to go back in the water...

In January 1998, Sydney's Daily Telegraph carried a front-page news story about American tourists Thomas and Eileen Lonergan, who had been mistakenly left behind on a diving tour on the edge of the Great Barrier Reef.

It took more than 48 hours before the boat's operators realised their error. The subsequent air and sea rescue operation failed to locate the couple.

Eileen's wetsuit was discovered, washed ashore, some time later, its fabric torn presumably by a shark.

These harrowing events provide the chilling inspiration for Chris Kentis's low budget thriller, which was the toast of this year's Sundance Film Festival.

Daniel (Daniel Travis) and his girlfriend Susan (Blanchard Ryan) attempt to escape their workaholic lives by heading off on holiday to a tropical resort for a week of relaxation and sunshine.

No mobile phones, no lap-tops, no contact with the office. As Daniel remarks: "Let's just try to remember, this is supposed to be a vacation".

Having spent the first day on the beach, Daniel and Susan make arrangements for a diving trip the next day with a crowd of fellow holiday-makers.

As experienced divers, they feel comfortable leaving the novices while they explore the seabed.

Daniel and Susan enjoy a magical half hour beneath the waves.

At 10.25am, when they resurface, they discover that the boat has left without them.

As they tread water trying to formulate a plan, scouring the horizon for the boat, a shark fin glides out of the clear, blue water...

Reportedly budgeted at less than £100,000, Open Water is a masterful exercise in sustained tension, playing on our primal fears of what lurks beneath the water.

Director Kentis and his wife and producer Laura Lau shot the film on weekends and holidays, and have not employed a single special effect or computer generated image.

Instead, the actors spent over 120 hours in the water, including the dramatic scenes with the grey reef sharks and bull sharks.

At times the suspense is unbearable and Travis and Ryan effortlessly convey the terror and desperation that take hold of their characters as the hours pass by.

Underwater photography is great, especially the shots from the couple's point of view.

Rating: 8/10