Uh... ALL the water?
Including sake, and presumably all forms of drink?
That's... Pretty unforgivable.
Unless you're running on an extreme case of Blue and Orange Morality there's really no justifying that. It also means the incident would have to be solved quickly, human bodies can survive for a while without sunlight or if spring comes a little late, but if someone happened to be doing any strenuous activity and suddenly found out they had nothing to re-hydrate with, there could be serious problems, even deaths.
I'd argue it's worse than anything even Seija did, at least she was thinking on backwards morality so everything seemed right to her.