sorry my descriptions are a bit useless in-game, im actually updating them now for next patch.

showers are fast and use about 65L of water to get 100% clean, so best used for productive colonies or when you have limited water supply, adv showers use twice as much water but are twice as fast to clean, provide more comfort, and in the next patch will heat water on demand so you wont need a boiler.

baths use 190 litres of water, are very slow to use forcing the pawn to stay in it for the full duration even after they are 100% clean, but provide a lot of comfort and joy, and in the next patch you can heat them using a campfire so you can have a very basic setup. So it will be for early game to replace buckets, or late game for luxury.

basins are for preventing disease, when a pawn uses a toilet there is a chance for them to pick up some bacteria depending on how filthy the room is, by washing their hands it clears it, if they don't wash their hands they have a chance to contract a hygiene related disease, if the water supply they use to wash their hands is contaminated then it will infect them with bacteria instead of cleaning it. In the next patch i'll be adding some more stuff to do with water quality to expand on that.

Does the shower need an open tile directly in front of it? Or can I have three showers right next to each other with a door in front of two of them? (so the colonists would enter from the side of them?

Any maluses for placing treatment plants inside?