Medieval beer was used in place of water and was rated at 2% or less. That was the beer people would drink 1300 liter per year. It was the common way to sanitize water, as the river was the source of clean water and the dumpster for dirty water at the same time.
I think it's worth noting it was the process that sanitised the water, not the alcohol - even raw eggnog with much higher concentrations is considered acceptable for most people but risky for the at-risk, due to eggs being largely but not completely clean.
Also mostly a myth. e.g. they weren’t drinking water from the Thames in medieval London for instance but from the Tyburn river. The case in most cities was that you had stream and springs which were legally protected and you poured your waste downstream into some major river. Obviously shit happened and water sources got polluted from time to time with terrible consequences but they still drank water all the time.
