I'm unsure if this is true, but I've read that commoners in medieval Europe preferred drinking beer and/or wine over drinking regular water, because the former was considered safer than the latter.
This begs the question what medieval muslim commoners (for example in the MENA region) drank to quench their thirst, considering many muslim realms forbid the consumption of alcohol.
Ah yes, the Muslim Variation of the Medieval Water Thing. The whole premise is unfortunately flawed, as the VFAQ explains (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. For bonus on the MENA region, u/CptBuck here addresses how desert civilisations do for water.