By Caroline Briggs BBC News Online Entertainment

The stars of the film spent 120 hours in the water during filming

The makers and stars of shark movie Open Water have spoken about the terrors of the sea and their delight at the film's unexpected success.

Loosely based on the true story of two scuba divers left behind by a boat crew, Open Water speculates on the shark-infested nightmare that followed.

It is a simple premise, but one whose petrifying plausibility caused ripples at this year's Sundance Film Festival.

The low-budget, docu-style film has even been likened to The Blair Witch Project, with others calling Open Water a Jaws for the 21st Century.

Husband and wife film-makers Chris Kentis and Laura Lau joined the two stars of the film, Blanchard Ryan and Daniel Travis, at a press conference in London.

I certainly never would have got into the water... if I wasn't being very, very well looked after Blanchard Ryan

In Open Water, Travis and Ryan play Daniel and Susan - the workaholic couple who go scuba diving while on a holiday and accidentally get stranded in shark-infested seas.

The man-versus-nature feel of the film exposes the fragility of being human in the inhospitable open seas and renders senseless the everyday tensions that have manifested in Susan and Daniel's long-term relationship.

The fear is made more real by the knowledge that, unlike the 1975 classic Jaws which used a mechanical fish, Open Water relied on real-life sharks.

Chain mail

Kentis and Lau worked with leading shark experts in the Bahamas who introduced them to a shoal who were used to contact with humans.

Bits of bloody tuna were slipped into the sea to encourage the sharks and get them worked up for filming.

It was an experience Ryan admitted she found frightening, despite the protection of chain mail under their wetsuits.

No special effects were used during the making of Open Water

"It's dangerous, they [sharks] are wild animals, and something could have gone wrong but the odds were very, very good that they wouldn't," she said.

"I certainly never would have got into the water - not that I would have admitted it at the time - if I wasn't being very, very well looked after.

"I was scared, but I knew we were safe. Sharks are very big muscular animals and they are scary."

The only injury during the days of filming in the open seas - 20 miles from shore - was when a barracuda bit Ryan on the hand.

"I still have the scar," she said proudly, "but it's not as impressive as it was."

But, as Kentis explained, the filming schedule was fitted around the work with the sharks to keep risks to a minimum.

"All the emotional stuff, the screaming and splashing around was done weeks later after we'd finished working with the sharks, as a safety precaution."

Low-budget

The low-budget nature of Open Water meant the entire movie was filmed piece-meal during holidays and spare weekends.

Writer and director Kentis edited the streams of footage in the hours before and after his full-time job.

Lau said: "We shot so much footage, we knew it was going to be an editing feat.

"Chris spent a lot of time in the 'Cloffice', as we called it, which was basically a closet turned into his office.

"Eventually we moved apartments and got him a little room."

Kentis said the comparisons the Blair Witch Project were not completely unwelcome

He admitted the low-budget nature of the film, the use of unknown actors and the docu-style filming meant the parallels were there to be seen.

"It's flattering, but we did not set out to make a film in the same genre," he said.

"It was not our intention to make a horror film at all."

Kentis and Lau said they had plans to make another film

And like The Blair Witch Project in 1999, Open Water was buoyed up by a good reception at the Sundance Film Festival in January.

All four said the film's success had exceeded their expectations.

Kentis said: "Getting into the Sundance was amazing, but you just don't know how a film is going to be received.

"So to be the first film to be sold out at the festival was mind-blowing."

And Travis, who had a few small television roles prior to making the film, admitted his hopes for the film extended to "a couple of scenes to use in our show reels".

The two unknown actors, who were friends before Open Water, said they were hopeful the success of Open Water meant more film offers would flood their way.

"We have been so busy trying to promote this, it is a question of getting back to Los Angeles and collecting ourselves," said Travis, who is now a keen scuba diver.

"I certainly wouldn't mind going into the water again."

Ryan was less forthcoming: "All my future roles are going to be indoors," she said succinctly.