Since the water a thousand years ago was likely to give you typhoid along with other things, they drank beer and wine. But since alcohol dehydrates you, were they just poorly hydrated all the time?
Ah, The Medieval Water Thing. It's not as bad as popular imagination believes it is. The Water Thing is in the VFAQ, specifically this section (Middle Ages, then Health & Hygiene, in case your browser doesn't take you directly there). You'll want the first two answers, by u/sunagainstgold and u/Qweniden.