Communal baths feature in many ancient (and modern) cultures, including Rome and Japan. Consider including one as a sub-level in your dungeon.
Although one group of monsters might secure exclusive use of the baths for a time, eventually various dungeon factions will contend for control over or at least use of the baths. Perhaps one dungeon faction runs the baths as a commercial venture catering to a multitude of dungeon denizens. Politics and intrigue might lure to the water even monsters indisposed to bathe for the sake of hygiene.
(This is for the A-Z Blogger Challenge.)