In the early-morning chill, the dock is icy and the water is slate blue. The boat's engine grumbles, the palm trees look tattered, and I shiver in my too-short, too-baggy wetsuit. It's Christmastime in King's Bay, Florida. Dress warmly and bring a snorkel.

I feel crazy going swimming in this weather. But when the boat leaves the dock and I look across the water, I can see that the 70°F surface is steaming, fueled by hot springs seeping up through the river mud. I jump into the warm water, and as soon as I put my head under, I see manatees.

"We live our whole lives around disguised animal thoughts."—Barbara Kingsolver

They congregate here every winter, drawn to the heat. A line of buoys marks off the area where we cannot go, and there, through the murk, the resting adult manatees look like Tootsie Rolls the color of cement. The curious young ones venture out to where we visitors float.

Despite their size, the manatees swim with a still, eerie grace. As we slowly explore the channel, a half-dozen calves nip the boat's mooring lines, as if trying to floss their gums. They nibble at our plastic fins with mouths that are all lip.

A young manatee swims between my legs. Its eyes are small and dark, set like raisins in its wrinkled face. Another manatee rolls over, its front flippers moving and bending like arms. Their skin is cracked leather, covered with sensitive, wiry hairs and a thin film of algae. They are quiet. They orbit us languidly, seeming to enjoy the company.

I swim upriver into clearer, colder water, until the chill drives me back onto the boat. The sun is up, the frost has melted, and I can still see the manatees when they breathe, rising from the deep like dark islands.