Green’s Dictionary of Slang

a person who swims in the nude.

1971		Tuscaloosa (AL) News 5 Sept. 7A: The skinny-dippers are a nocturnal breed and come out only at night.
1986		L. Heinemann Paco’s Story (1987) 168: Listening to the skinny-dippers horsing around in the sand-bottomed shallows.
1992		R.C. Cruz Straight Outta Compton 69: Billy saw the ice cubes dance in the alcohol like skinny-dippers playing in the surface water of a pool.
2004		C. Hiaasen Skinny Dip 327: He’s a hotshot storm trooper for the Park Service, spying on innocent skinny-dippers.