Extremely tiny, baby crabs usually lain offshore but washed in by storms, etc. They attach themselves via their tiny claws to the bodies of unsuspecting swimmers. Painful and annoying, especially when there are thousands of them. Even worse once they get into ones swimsuit and entangled in pubic/butt hair.
Swimmer: Mr. lifeguard, something in the water is biting my testicles
Lifeguard: sounds like sea lice