water from a water tub becomes contaminated once its been used too many times, then pawns wont drink from it, if they are getting diarhea a lot then it could be several problems, like your rooms are filthy, they aren't washing enough because you don't have enough water tubs to service everyone, they aren't washing their hands after using toilets, normal things like that

Thanks for the quick reply! It's early game and there's only 4 colonists so I figured 1 tub would be enough, guess I'll expand. I do wish I could stop them from drinking that water though, regardless of the diarhea it bothers my perfectionist side :)