With baited breath

Part of what makes horror work is empathy.

How many times have you told yourself “I would never walk down that dark hallway!” even as you watch some seemingly stupid character in a horror film do exactly that.

After watching “Open Water,” you’re likely to tell yourself, “I would never go on a scuba-diving trip!”

One of those little movies that came out of this year’s Sundance Film Festival with a lot of buzz, “Open Water” is every vacationer’s nightmare. A cross between “Touching the Void” and “Jaws,” it compensates for its made-on-the-cheap feel (the estimated budget was a mere $130,000) by grabbing our throats and, slowly but surely, stopping our collective breath.

Work-obsessed Susan (Blanchard Ryan) and Daniel (Daniel Travis) are control freaks who just can’t put down either cell phone or laptop computer. Work has estranged them so much that their vacation is meant to serve a dual purpose: Not only is it a way for each to relieve stress, but it’s a chance for them to repair their relationship.

Which they do. Unfortunately, they’re so busy reconnecting on a diving excursion that they don’t make an impression on their fellow divers. And when they take a bit too long to resurface, they discover that the boat has left them behind. Seems the dive master has miscounted and no one noticed that they were missing.

So begins their struggle. Not only is the current taking them farther out to sea, but they are too small to attract the boats they see in the distance or the occasional plane that flies overhead. They have no water, no food, night is coming and the ocean is getting progressively colder.

Then the sharks show up.

Written and directed by Chris Kentis, “Open Water” maintains a riveting sense of tension, even though two-thirds of its short (79-minute) running time is spent watching Susan and Daniel tread water. Kentis’ decisions may have been caused more by budget constraints than pure artistic choices, but the result is the same. That’s also true for the cinematography, which at Sundance was marred by that now-familiar washed-out digital video look.

But when that first fin pops up, aesthetic concerns are likely to evaporate as quickly as sweat on a bald man’s scalp.

And despite being relative unknowns, Ryan and Travis manage to keep their respective characters sympathetic even as they bicker, play the blame game and grow increasingly aware that they have no control in an environment that views them as little more than fish bait.

When the end comes, it does so naturally. Even so, it is no less shocking.