“Actor and actress needed. Must have scuba diving certification. Must be willing to swim in close proximity to sharks. Must do own hair and makeup. Long hours, no pay. Nudity required.”

That’s not exactly how the casting ads read when filmmakers Chris Kentis and Laura Lau went looking for the stars of their self-financed project, “Open Water,” but it’s close.

And still, the lines of struggling New York actors and actresses who showed up for auditions were daunting.

“I didn’t realize, until we went through this process, how tough it is out there for actors,” says Kentis, who found his leads – newcomers Daniel Travis and Blanchard Ryan – after several weeks and scores of interviews. “The turnout was amazing.”

Shot on weekends and vacations over a two-year period, “Open Water” is about a couple on holiday who go scuba diving and get left behind by their boat. Alone in the ocean, 20 miles from land, they have to figure out what to do and how to cope until the crew members realize their mistake and come back to pick them up.

The deep-sea suspenser, made on digital video for a meager $130,000, debuted at the Sundance Film Festival in January. On the first night, offers started pouring in. Lions Gate Films purchased the husband-and-wife team’s “home movie” for $2.5 million.

“Daniel and Blanchard did it all on deferred payment,” reports Kentis, on the phone from his home in Brooklyn. “We were very happy to be able to pay them once we sold the movie. Very, very happy.”

A film editor who makes his living – or made his living – cutting trailers for the Hollywood studios, Kentis, 40, and his wife, also 40, are scuba divers themselves. The inspiration for “Open Water” came from reading an article about an Australian couple abandoned by their dive boat and left floating in the ocean. He wrote the script and directed. She produced. They both shot it.

“Our hope, if the movie was going to work, was that audiences, once the couple are in the water, would put themselves in the place of these characters,” says Kentis. “That they would … start asking themselves the vital questions, like what would I do in this situation?”

On the week of their film’s theatrical release, Kentis and Lau are still a bit awed by the turn of events since Sundance: the press interviews, the marketing meetings, the big-deal New York premiere, and the buzz that “Open Water” is the next “Blair Witch Project” – an out-of-nowhere, no-budget indie ready to rake in millions. Comparisons have also been made to another peril-at-sea picture, a little thing called “Jaws.”

“Ironically, we never saw this as a shark movie, never saw this as a horror movie in any way,” he says. “These were elements that existed in a true story that we wanted to tell. And if we were going to tell the story properly, sharks eventually were going to become part of that mix. …

“But I’m a little wary. We came out of Sundance and started hearing these comparisons. And it’s incredibly flattering. It’s also really frightening, because those are two record-breaking films, and our film is very different.

“We just hope that audiences, rather than go in expecting a kind of ‘Blair Witch’-meets-‘Jaws,’ come to our movie with an open mind, and see it on its own terms.”