(covers information from several alternate timelines)
In 2369, while under the influence of the Saltah'na energy spheres, Lieutenant Jadzia Dax told Major Kira Nerys "You know what they say - put the shoe on the right foot first, but put the left foot first into the bathtub", which seemed to confuse Kira. (DS9: "Dramatis Personae")
In 2370, Ensign Melora Pazlar explained to Doctor Julian Bashir that experiencing low gravity was "like slipping into a hot tub at the end of the day", to which Bashir quipped, "I'm a shower man myself". (DS9: "Melora")
Later the same year, Captain Jean-Luc Picard and Lieutenant Commander Data discovered Counselor Deanna Troi in her bath tub during her transformation into an amphibious creature, a result of Barclay's Protomorphosis Syndrome. (TNG: "Genesis")
In 2372, Captain Kathryn Janeway and Commander Chakotay became stranded on a planet they named, "New Earth." During their time together, Chakotay built her a bath tub in which she greatly enjoyed bathing. She also had a bath tub in her quarters on board the USS Voyager, where, in 2378, she was surprised by Q, who appeared while she was bathing. (VOY: "Resolutions", "Q2")
Morn kept a hot tub of mud in his quarters on Deep Space 9 which he used as a mud bath as well as a place to sleep. After faking his own death in 2374, he left it, among other things, to Quark, who subsequently drained it empty, believing him to have hidden a stash of latinum there. (DS9: "Who Mourns for Morn?")
- In a deleted scene from the Star Trek: Deep Space Nine episode "Let He Who Is Without Sin...", Chase Masterson's character Leeta, originally naked, had a bath in a bath tub on Risa. This scene only appeared in the promo trailer for the episode.