I'm afraid your question rests on a false premise. Medieval water cleanliness was Serious Business, and the notion that they all drank alcoholic drinks to make water safe is pop-cultural nonsense.

I shall direct you first to the VFAQ (Middle Ages, subsection Health and Hygiene, in case your browser doesn't go there immediately), in particular the answers from u/sunagainstgold and u/Qweniden.

To illustrate just how Serious Business water was for the Medieval era, more u/sunagainstgold on the incidents she touches on in her VFAQ post:

• On the father advising his sons to drink the well-water boiled;
• On the woman in Siena who was flayed alive for poisoning a fountain;
• And several more illustrative accounts of the Medieval attitude towards water safety.

On the specific matter of Fetal Alcohol Syndrome in the Medieval Era, see next post.