Janeway's bath tub
Morn's hot mud tub
A bath tub is a place for cleaning and relaxing, filled with water. It is mostly part of a bathroom.
Deanna Troi was found in her bath tub when turning back into an amphibic creature with gills due to Barclay's Protomorphosis Syndrome. (TNG: "Genesis")
Intendant Kira Nerys had a bath tub in her quarters aboard Terok Nor in the mirror universe. She enjoyed this bath and had the help of two Vulcan servants. (DS9: "Crossover")
Morn kept a hot tub of mud in his quarters aboard Deep Space 9. (DS9: "Who Mourns for Morn?")
Unable to find a cure for their illness, Captain Kathryn Janeway enjoyed a bath in her bath tub, while stranded on New Earth in 2372. Chakotay built this one for her. (VOY: "Resolutions")
Janeway also had a bath tub aboard the USS Voyager and in 2377 she was surprised by Q who teleportated directly into her tub. (VOY: "Q2")
Counselor Deanna Troi and Commander William T. Riker had a bath together in a bath tub while the USS Enterprise-E was in the Briar Patch. Troi shaved Riker with a razorblade. (Star Trek: Insurrection)
Background
Leeta in the bath tub
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.