Skinny dipping was the practice of swimming naked, in natural bodies of water, swimming pools, or hot tubs.
During a trip to Risa in 2373, Quark suggested that he, Julian Bashir, and Jadzia Dax should go skinny dipping in the Risian hot springs. Bashir agreed with the Ferengi, something that surprised him. (DS9: "Let He Who Is Without Sin...")