The thing is that water was not really safe to drink, no matter what these people may tell you. There's a reason there are huge aid campaigns all over the Third World to ensure people have access to safe water. Some water is relatively safe to drink, but even in the wild you can get giardasis and other problems from drinking it. The more human beings are nearby, the more of an issue it becomes.

In the time before cars transporting water was not easy, so people usually had to get water from the nearest source. Wells were not necessarily safe, especially because both humans and animals tended to shit pretty much everywhere. Even today well water is not necessarily safe.

But did people know that drinking water was unsafe? Evidence on that is contradictory. They were certainly aware that some kinds of water was safer than others.

And was this why people drank beer instead? Not clear at all. It's completely possible they did it simply because they wanted to, although it was seen as healthy. That was because of the calories, though.

In many places they did not drink beer, however. Scotland and Norway drank blaand (a whey drink), and Eastern Europe drank a lot of kvass. Fermented birch sap and a drink from juniper berries were common, too. Not to mention a weird drink known as rostdrikke/taar/etc depending on language (takes too long to explain).

What I find interesting about this is that nobody seems to care to really dive into the details and describe the situation as it actually was. I realize it's a lot of work, but still.