while moving around the world map they will randomly find water sources to wash with and the bladder will reset when it gets low, and in encounter maps you just let pawns take a dump wherever on the ground in the map like they would have to irl, the game forces you to leave maps quickly so it shouldn't be a major issue, and if its a raid they hold it in while drafted

There's an item called a "piss point".
Can't remember if it's part of the Bad Hygiene or not.
I think it is?
Anyway, it's something you can put down when you reach your destination for your colonists to use without them getting all sad that they had to take a dump out in the open.
You can find it in the Misc tab.
It SHOULD be with the hygiene stuff, I know...but it isn't. :P
It's just a little yellow square.
I think they're great! :D