When I wrote The Wrong Sword I assumed, like about a million other writers of historical farce/fantasy/drama/baloney, that folks back then preferred beer and wine to water because water could be "iffy." Turns out that's not true! Our ancestors were NOT morons. And here's the blog post to prove it:
It makes sense if you actually sit down and think about it. Contaminated water is a bugaboo of human occupation, especially settled, urban-ish occupation. We have a tendency to crap where we eat (and drink), after all. And in Medieval Europe, with far fewer people and far more untended land and unpolluted watersheds, the likelihood of finding clean water - from a spring, a well, a stream or even a sufficiently clean pond - was that much greater.
And there it is!