To create a dwarven bathtub, just make a channel and fill it with 3/7 water via a pond zone, being careful to remove or deactivate the zone before it gets too full. One of these across each fortress entrance, each cavern entrance and the hospital entrance will go a long way towards keeping contamination under control. If you need to clean specific dwarves, for example those giving you 'clean self' cancellation spam due to the currentv.31.18 soap bug, create a lever that does nothing on the far side of a Dwarven Bathtub, assign the lever to that dwarf (this requires a Manager noble), and order the lever to be pulled. This also works to clean contaminated pets, if you find the pet owner and order them to pull the lever. Unowned animals, of course, can just be butchered or thrown into the tub by changing your pond zone to a pit and dropping them in.
Why Dwarfs Don't Bathe Outside
There are several things to watch out for with outdoor bathtubs. First, they need a roof over them to prevent evaporation. Just build some walls on the side, a ramp next to the walls, and then a floor over the top of them. Once all the tiles with water show up as Inside, you shouldn't have to worry about evaporation any more. Freezing biomes are more of a worry, because you do not want a tub that will freeze over. Make sure any place you build one is Subterranean and that it stays that way, or you dwarves may freeze to death if they're in the tub at the wrong time.