Jagi56's initial comment seemed ludicrous to me (You couldn't live without drinking water, and alcohol is actually generally dehydrating) until I read further on this page, so let's see if I understand right. The beer they drank was barely alcoholic and the main purpose of the alcohol was actually to clean the water by killing bacteria?

Depend, the water look good to me, no I would not drink anything downstream from an city or near the hot springs. Would also be careful about standing water especially in dungeons.
Little water pollutions as the population is low.

Might say that the lack of drinkable water is of the same reason as it's a lack of bottled air. You don't care about air unless you are diving or in space. You care about water in the desert not where it's everywhere.

I ended up looking this up when I made my needs mod. Turns out being drunk does cause you to process water more quickly, but there's enough water in even hard alcohol to offset it. So that's how I handled it - alcoholic beverages had whatever their normal number of ounces of water would be irl, but your rate of dehydration increased as your BAC did.

They must have non-alcoholic mead too We have that, my mom makes it herself and it's the only kind I ever drink.

Correct, the ancient brew was about 1.2% alcohol by vol (the standard today is 5% for regular and 3.5% for lite beer) but that's not even the best part. Watch the documentary i linked to and it tells you how (unknown to ancient man) that the recipe for brewing beer actually produces a very beneficial antibiotic(only very recently discovered) that greatly increased the avg life span of anyone that consumed it on a daily basis.