We humans are funny creatures. Anything that frightens us gets transformed into an Evil Force bent on our destruction. We turn a snake into Satan, death into a cloaked ghoul with a scythe. Our paranoia ascribes human traits like sadism and vengeance to other animals, making them into mirrors of ourselves. That’s why Jurassic Park’s velociraptors kill for sport; why the titular Orca, The Killer Whale chases after the fisherman who hurt her. But in Open Water, there are no monsters, no showdowns between good and evil. There are only living things going about their business, which makes the movie even more chilling.

Less-than-happily married Daniel (Daniel Travis) and Susan (Blanchard Ryan) are going on vacation. Being certified scuba divers, they sign up for a diving tour off a tropical island. The dive itself is wonderful: they go exploring by themselves, pet eels and wave at barracudas. But when the couple resurface, they are alone.

At first, our heroes are bemused. Are they early? Are they in the right spot? (Of course they are, says Daniel. There’s a coral head right below them). As the hours tick by, Daniel and Susan try to comfort themselves with bravado-tinged logic: “They have to know we’re missing…we have their tanks!” But their tour boat doesn’t come, and the sharks are getting hungry.

Writer/director Chris Kentis and producer Laura Lau got the idea for Open Water from true stories in diving magazines of divers lost at sea. Having already made Grind, Kentis and Lau were intrigued by digital video: a technology that’s getting cheaper all the time. This technology has shaped the film, and in Open Water’s case, that’s not a bad thing. Its little-more-than-home-video style makes it seem more like a documentary than a movie. And we, in turn, feel as if we’re watching real people fight for their lives.

Kentis’s script also contributes to the illusion that Daniel and Susan are real people. Their speech is peppered with the trivial observations and repetitive statements that most of us make (“Did you see that on Shark Week?” “No. History Channel--‘Disasters at Sea’”), without veering into the unscripted inanity of, say, The Blair Witch Project. Even their stupid arguments ring true. Although they’re stuck in the middle of the ocean and could die at any time, Daniel and Susan still bicker over whose fault their predicament is. Does it really matter if he made them miss their boat or if her job somehow made them spend their vacation here? (No, I don’t understand that argument, either). It’s exasperating to watch, but so very familiar.

In fact, “exasperating” is a pretty good description of Open Water’s protagonists. They love each other very much, but they never really talk. When we first see them, they’re each talking on the phone: to co-workers, not each other. Susan talks about their schedule while Daniel gazes lovingly at the laptop. And when Daniel wants to make love, Susan isn’t in the mood. Even their “playful” ribbing leaks anger that they won’t or can’t express. Like some neurotic who inherits a haunted house, the couple’s personal problems leave them vulnerable to the awful things that happen.

Part of what makes those things so awful in Open Water is that they occur without many of the bells and whistles that tell us “it’s only a movie.” While the film has some musical set-pieces (recordings of island music from the 1940s and ‘50s that have been touched up wonderfully by Graeme Revell) it doesn’t use music as a cue to tell us what’s coming next. There are no happy trumpet-swells when boats are on the horizon; no cellos or screeching violins when sharks circle. The filmmakers occasionally use this amusia to mischievous advantage. At one point, our heroes’ ceaseless bobbing was broken by a very deep hum in the distance. “Thank God!” I thought. “A boat!” But as the humming got louder, I realized it was just deep-voiced men singing.

Not only do the filmmakers cut out musical cues; they also eschew mechanical sharks and other particularly special effects. All of the sea creatures in the movie are live animals, not props. Chris Kentis has said that he wanted to capture sharks’ natural behavior, and I think he’s done it. Open Water’s sharks are not demons; not genetically engineered superbeasts destroying their own mad scientist god. They’re just…sharks. As Susan floats on her back, a brownish shark clan swims underneath her, just below the water’s surface. I gasp, put my hand over my mouth and think, “Wow.” Even in their menace, the sharks are beautiful.

Lion’s Gate really did a nice job with this disc. It’s not quite as impressive as their Cabin Fever disc, but there’s still a lot of extras for fans to sink their teeth into. Available in both fullscreen and widescreen versions, it sports two commentary tracks--one with Kentis and Lau, the other with stars Daniel Travis and Blanchard Ryan--a “making-of” featurette, some bonus footage showing how the actors suited up with chainmail to swim with the sharks, and some deleted scenes. Many of these scenes explore Daniel’s and Susan’s relationship, and the movie is richer with them. But my favorite feature is “The Indie Essentials: A Filmmaker’s Guide to Gearing up for a Marketable Movie.” In it, Lion’s Gate executives talk about what drew them to Open Water in the first place.

There are plenty of sharks in Open Water, but it’s not really a shark movie. Its themes reach far beyond deadly animals. This film is about the accident of the human condition. We’re just a quirk of evolution, cogs in the natural machine. If that machine mauls or destroys us, the world doesn’t stop to mourn.