THE DIRECTOR Chris Kentis could probably have saved Steven Spielberg a lot of time and money. Spielberg’s problems with the malfunctioning mechanical shark on Jaws almost sank his entire film. Kentis, however, came up with an alternative, cost-effective solution to shoot his ocean-bound thriller Open Water, in which husband and wife scuba divers are accidentally left behind by their tour boat in shark-infested Caribbean waters: real sharks.

“I think there’s a tendency to overuse computer-generated or special effects today,” Kentis explains, “and that takes me out of the experience of following the movie. The sharks are an element to this true story, so it makes sense that, if you’re going to capture it in the most realistic way possible, you use the real thing.”

The true story referred to is of an American couple stranded by their dive boat off the Great Barrier Reef in 1998. Their bodies were never found. Using this outline rather than specific details, Kentis and his wife, the producer Laura Lau, themselves experienced divers and emboldened by the DIY, Dogme-led movement, immersed themselves in the project.

An entirely self-funded film (the lack of expensive CGI wasn’t simply an aesthetic choice), Kentis, Lau and a crew of just two dived in alongside game actors Blanchard Ryan and Daniel Travis, training their DV cameras on them (the pixellated format actually makes it harder to differentiate dorsal fins from choppy waves) as dozens of 8ft grey reef and bull sharks circled.

Safety was paramount. The shark-wrangler Stuart Cove is a Bond movie veteran and the Bahamian shark population employed were used to divers. But Kentis admits: “All it takes is one little accident and with a shark it’s a very big accident.” Both actors wore chain mesh under their wetsuits, but Ryan’s fears were only allayed up to a point. “I thought I could handle being in the water with a shark,” she confides. “I didn’t know I was going to have 50 of them banging into me, with tuna being lobbed at my head.”

Ultimately it wasn’t the two-day stint with Cove’s “union sharks”, as they dubbed them, that most unnerved the actress. “With those, I knew they were there, I was completely prepared for them,” Ryan recalls. “But we were also in the water for 30 other days from morning till night, and every once in a while a shark would come up out of nowhere. Those ones would just paralyse me.” Now, of course, she can laugh. “It was good method acting, you know? Anyone who says the girl didn’t look scared during that movie is lying!” Fortunately there were no serious accidents at sea, despite Ryan being nipped on the very first day of filming by a barracuda. More dangerous was a freak volleyball accident that sidelined Travis for a year. “That was the scariest part of the whole process for me, phoning Chris from the hospital,” the actor shudders. Kentis and Lau’s commitment and financial independence meant that they waited for his return — hardly standard Hollywood practice.

“This project was never about anything to do with Hollywood,” Kentis insists. “The whole reason that we wanted to do it all ourselves was that it gave us the freedom to play and experiment as film-makers.” Still, a successful Sundance Festival splash led to a multi-million-dollar studio feeding frenzy for a film that cost only $130,000 (£72,000). Canny marketing — “Jaws meets The Blair Witch Project” — may produce a sleeper hit but one industry’s delight is another’s PR disaster; Open Water is likely to do for scuba diving what Blair Witch did for camping.

Lau claims they’re “not out to indict the dive industry” and praises its safety record, yet in the next breath Kentis states: “We’ve got e-mails from divers who say the film’s plot ‘sounds like my story’.” Yet more than the sharks or any anti-diving scaremongering, it’s the way that the film taps into primal fears of the unknown and utter helplessness against the forces of nature that helps to make Open Water such a chilling experience.

• Open Water opens on September 9

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