In Germany, there is "case law" to that effect. Letting someone suffer of thirst is "unterlassene Hilfeleistung", roughly translated "neglecting to give aid". Not giving someone suffering of thirst any water is considered as an example in legal literature, however, I know of no real case where someone was convicted solely for that. And it isn't really straightforward to argue that a restaurant not serving tap water would fulfil the criteria of "unterlassene Hilfeleistung", at least because the usual restaurant-goer isn't suffering that badly.

Also, aforementioned aid doesn't have to be free, the person benefiting from your aid owes you the cost of that aid.