Hrm...sentient fungus...where have I seen this before?
(just kidding)

Well thought-out, good fluff, and mechanically balanced. Could you ask for anything more?

If they have to have so much water, would they even have a fire vulnerability? If they require 2x normal amounts of water, that would make them, like, 90-95% water. Much more than humans, and humans don't take extra damage from fire...