Delivering a yacht becomes an excuse for a laid-back tropical vacation–and then for a grueling, grisly fight to stay alive. I hereby dock The Reef half a star for unavoidably getting “Sloop John B” stuck in my head: This is the worst trip I’ve ever been on.

Andrew Traucki’s lean, unshowy film is effective in a restrained kind of way. It’s based on a true story, and it feels like one, which is both a ding on its structure and a compliment to its tone. Sometimes–as with the muted, stunned ending and its haunting loose end–that quality works in its favor; sometimes–when there’s nothing the characters can do about the shark except resign themselves to it and numbly wait to see who will make it out alive–that lessens its impact. It’s hard, though possible, to make a good horror film about people who are prepared to take on the horrors. It’s equally hard to make an exciting film where the characters are in so far over their heads that they’re essentially helpless.

With that in mind, The Reef is wise to not try to make its melancholy death swim propulsive. Instead, it captures–and magnifies–its characters’ exhaustion, making you feel the soreness and stiffness of their limbs and the paralysis of their terror and (eventually) grief. Since there’s not much they can do to fight back, the movie focuses on the resistance of their kindness, instead. There are tiny flare-ups of conflict as they attempt to swim from their wrecked yacht to the nearest island, but they usually sputter out. (It certainly helps that there aren’t many of them, and they all care about each other already.) The arguments aren’t as important as, say, a character with a bitten-off leg frantically urging the others to leave, to get away while they can, or another character bandaging someone’s bloodied foot without a word about what that blood will inevitably attract.

Actually, The Reef has a moving faith in human courage and decency, if not in the resilience of human flesh. Its natural world, sharks and all, doesn’t feel vindictive so much as uncaring: it’s doing its thing the way it has been for billions of years. Oceans are vast. Sharks are hungry. Coral is hard and sharp. Islands and reefs were not crafted for comfort or ease of access. But in the midst of this indifferent wilderness, you do occasionally get good people. They won’t last long, and in fact, being good sometimes makes their lives even shorter. But they’re there.

The Reef is streaming on Amazon Prime.