Some films are easy to summarise, and Open Water is one of them. You will probably hear many times over the next few months, "Open Water is Jaws meets The Blair Witch Project" -- a statement that doesn't flatter the movie  unreasonably, because Open Water is every bit as chilling and suspenseful as this comparison suggests.

The film is based on a true story, albeit loosely, as it follows a successful young couple preparing for a much needed holiday. While they pack, tension hangs in the air: Susan’s phone never stops ringing, and Daniel tries to persuade her not to pack a laptop. It’s clear that a diving holiday is just the kind of seclusion they need.

They arrive at the resort, and in the morning join a diving party which takes them 18 miles out into the ocean. The pair dive together, but when they resurface the boat has disappeared, after a mix-up with the headcount. They find themselves stranded in shark-infested waters, waiting for the boat to return.

Open Water, shot on handheld digital cameras, is a thrilling example of the power of digital video. The graininess of the format, combined with the wealth of natural lighting, give the film a stylish realism that makes the characters’ plight terrifyingly plausible. The film uses no special effects, so almost everything we see is real in some sense. The actors spent 120 hours in the water with up to fifty sharks at a time.

Despite the Jaws comparisons, sharks are really only a small part of the terror of this situation. Susan and Daniel have no control over where the currents take them, and are almost invisible in the water. The water isn’t cold enough to kill them, but they can be sure that unless the boat comes back, they will die in these waters one way or another. And all they can do is wait, as the sharks circle beneath them.

Open Water is a genuinely scary film, packed with suspense and psychological terror. The best shockers give us something new to fear -- I imagine divers will be paying extra-close attention to headcounts for the next few years.

(Released by Lions Gate Films and rated "R" for language and some nudity.)