Article by Tim O’Neil, medievalist:
Contrary to what is found all over the Internet on the subject, the most common drink was water, for the obvious reason: It’s free. Medieval villages and towns were built around sources of fresh water. This could be fresh running water, a spring or, in many cases, wells. All of these could easily provide fresh, disease- and impurity-free water; the idea that water from these sources would be the causes of disease and so had to be made into ale or beer is fanciful.
Where water was more likely to be contaminated, largely by tanning, slaughtering, or dying facilities, was in larger towns. But since medieval people were not idiots, they dealt with this in several ways. There were ordinances on where tanners and dyers could operate so that water for domestic use could be drawn from rivers and streams in the town to ensure the water was clean. And there were fines for contaminating areas of streams used for household consumption.
I can’t recall any sources I’ve read off-hand that would dispute this. Tim does cite Ian Mortimer as a source, a well-known medieval historian, so I would have to say this is probably accurate. It’s very interesting and contrary to what most people believe. Thoughts?