Hey, everyone knows that medieval people drank beer because they couldn't drink the water, right? "Wrong," says blogger chezjim on Les Leftovers.
The idea that Medieval people drank beer or wine to avoid drinking bad water is so established that even some very serious scholars see no reason to document or defend it; they simply repeat it as a settled truth. In fact, if no one ever documents the idea, it is for a very simple reason: it's not true.
Not only are there specific – and very casual – mentions of people drinking water all through the Medieval era, but there seems to be no evidence that they thought of it as unhealthy except when (as today) it overtly appeared so. Doctors had slightly more nuanced views, but certainly neither recommended against drinking water in general nor using alcohol to avoid it.