When you’re swimming in the middle of the sea, surrounded by about a dozen live Caribbean reef sharks, there are a couple of rules to keep in mind:

1) Don’t panic.

2) Whatever happens, just try to avoid looking like food.

That’s what the stars of the Sundance hit “Open Water” learned while filming their survival-among-sharks movie, which hits New York and Los Angeles theaters on Friday.

“You don’t want to splash around,” says Daniel Travis, who plays half of a scuba-diving couple who get stranded mid-ocean after their tour boat leaves them behind.

“That’s what a wounded fish does, and it gets the sharks excited.”

That was important knowledge for Travis and the rest of the tiny “Open Water” crew, since they didn’t use a single special-effects shot in the movie.

Instead, they went to the Bahamas and filmed with real 9-foot-long, 500-pound reef sharks. With help from a shark wrangler, they attracted the animals by tossing bloody fish bits into the water – then hopped into the middle of the frenzy to do their scene.

That’s method acting in the extreme, and it was almost a bit too much for Travis’s co-star, Blanchard Ryan.

“I got overwhelmed a couple of times,” Ryan admits. “There would be all this blood in the water, and I was getting bumped too much, and I’d be like, ‘Oh my God!’ and start hysterically crying, ‘I can’t do this anymore!’ ”

Fortunately, Travis and Ryan hung in there, because now the truly scary “Open Water” is the talk of Hollywood – a potential star-maker, not just for the actors but for Chris Kentis and Laura Lau, the husband-and-wife team from Brooklyn Heights who singlehandedly wrote, produced and directed the movie with $120,000 of their own money.

“Open Water” sold for about $2.5 million at Sundance.

Over the next few weeks, the film distributor Lions Gate will release the movie around the world, with the hope that it will catch on with mainstream audiences, just like the 1999 indie phenomenon “The Blair Witch Project,” which grossed $140 million in the U.S. alone.

While “Open Water” doesn’t have the groundbreaking Internet-buzz marketing campaign of “Blair Witch,” most critics say that it’s the better movie.

It’s as much a survival story in the tradition of “Into Thin Air” as a horror movie about sharks.

“It’s about isolation, and people facing the elements,” Kentis says. “We tend to forget the forces of nature – and our place in the food chain.”

The movie is based on a true story about a real-life young American couple, Tom and Eileen Lonergan, who got left behind while scuba diving on Australia’s Great Barrier Reef in 1998, when their tour-boat operator accidently botched the headcount.

Kentis and Lau first read about the Lonergans six years ago, in a dive magazine.

“Chris just freaked out when he heard that story,” Lau recalls.

“He was going around for days saying, ‘That’s absolutely the worst thing that could happen to anyone.'”

In other words, it was a perfect idea for a movie.

Kentis and Lau had already made a film at that point – 1997’s “Grind,” a drama starring then-unknowns Billy Crudup and Amanda Peet that made the festival circuit and got a small theatrical release.

After “Grind,” Kentis went back to his day job as an editor at the SoHo production company Giaronomo, which produces trailers for some of Hollywood’s biggest blockbusters, including “The Matrix” and “Spider-Man.”

Lau, meanwhile, stayed at home to raise their infant daughter, Sabrina.

But they saved up their money to do another movie and settled on the diving story because they thought it would look good shot documentary-style, with hand-held video cameras.

Kentis and Lau wound up shooting the movie themselves, using a pair of $3,000 Sony cameras they bought at the B&H store on 34th Street.

They discovered Travis and Ryan at auditions in 2001, when both were struggling Manhattan theater actors.

At the time, both were best known for small roles on “Sex and the City.” In a 2000 episode, Ryan played a Vera Wang saleswoman who helped Charlotte try on wedding dresses. A year later, Travis played “Captain Crunch,” the guy who hit on Miranda at the Crunch health club.

Travis and Ryan are also old friends, who first met about seven years ago, when they had small parts on “All My Children.”

While they have never dated – “we always had other boyfriends or girlfriends,” Ryan says – the pair got close enough that Ryan used to take Travis out for his birthday every year.

“When I walked into callbacks and saw Daniel, I was like, ‘Uh-oh, here’s trouble!,’ ” recalls Ryan. While they were filming the shark scenes, Ryan had to lean on her old bud. “I kept feeling something bumping me on my legs, and I’d go, ‘Daniel, stop kicking me!,’ ” Ryan recalls.

“He would apologize profusely – which was very nice, since it wasn’t him bumping into me.”

Not only is Ryan a self-described “big chicken,” but on the first day of filming she got nipped by a barracuda.

“That didn’t help my confidence,” she admits.

Travis actually liked diving with the sharks, and even swam with them when the cameras were off.

“I just figured they liked tuna more than us,” he says.

But the biggest daredevil of the bunch was Kentis, who filmed much of the movie from the water, fighting strong currents and sharks who wanted to eat his camera.

“Chris was crazy with the sharks,” Ryan recalls. “They were biting his camera, and he would bang them on the nose to wrench it out of their mouths.”

Still, the “Open Water” crew members were as safe as they could be, since they were working with the experienced shark wrangler Stuart Cove, who has arranged shark stunts for dozens of movies, including James Bond flicks.

“We weren’t going to mess around with safety,” Kentis says. “We spent the money to get the top guy.”

Cove lives in the Bahamas and works with two populations of wild Caribbean reef sharks. He knows the animals so well that he takes thousands of recreational divers to swim with them each year.

Cove has been in hairy situations in the past – for example, while filming the 2001 documentary “Ocean Men,” a piece of bait landed on his head and a shark took a bite out of his scalp.

But he insists that swimming with sharks is perfectly safe. “We can create a feeding frenzy and actually get into the middle of it,” he says.

“There will be 30 to 40 sharks eating like crazy, but they’re so agile that they’ll come between your legs and grab the fish without touching you.”

That may be, but it was still plenty scary for Ryan.

“I’m not sure how I did it,” she says. “I think some other girl took over my body for this film.”