(covers information from several alternate timelines)
A bathing suit was a special garment used for swimming.
Kristin wore a bathing suit while using Holodeck Program 47-C in 2368. Upon losing her memory along with the rest of the USS Enterprise-D's crew, she bemoaned the fact that her attire didn't give any clues to her identity, and asked Dr Crusher for a change of clothes, at least until she found the swimming pool. (TNG: "Conundrum")
Jadzia Dax wore a bathing suit when visiting Risa in 2373. Worf said that she was even more beautiful wearing it than a protostar cluster he saw in the Gamma Quadrant. (DS9: "Let He Who Is Without Sin...").
In 2377, B'Elanna Torres was apprehensive about The Doctor's offer to join her in the Gedi Prime holoprogram, saying "Somehow I don't think that would add to the mood." He replied with, "Don't be so sure. You haven't seen me in a bathing suit." (VOY: "Drive")