(covers information from several alternate timelines)
Waiter wearing a bathing suit
Worf holding up his bathing suit
A bathing suit or swimsuit was a special garment used for swimming. One type was the bikini.
A photograph of Lucille Harewood taken when she was well showed her sitting on a beach wearing a bathing suit. (Star Trek Into Darkness)
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...").
B'Elanna Torres and Kes wore bathing suits when visiting the Paxau Resort holoprogram in 2373. (VOY: "Warlord", "Alter Ego").
B'Elanna's bathing suit was later sold off on the It's A Wrap! sale and auction on eBay. [1]
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")