LOXAHATCHEE GROVES, Fla.—On a recent Friday morning, Jessi Bartoletti arrived at the Sunsport Gardens Nudist Resort here in a T-shirt and shorts.

By evening, the 19-year-old had stripped down to a string of purple Mardi Gras beads and was dancing around a bonfire with about 200 young nudists, many of them first-timers.

"I don't think I've...