I was watching Pawn Stars once and heard it said that way back when, early peoples' water supplies became tainted and undrinkable. After a while, some people just started drinking beer because it was the only fluid around that wouldn't kill them. How true is this? If it is, could someone elaborate please? I'd like to know more.
So somebody did ask a sort of similar question [here] (http://www.reddit.com/r/AskHistorians/comments/1oumew/i_watched_how_beer_saved_the_world_and_most_of_it/), but it doesn't really quite answer your question, and honestly doesn't answer the original question on that thread either. I've heard similar claims too and would love a better answer.
Edit: Perhaps more related to your question: http://www.reddit.com/r/AskHistorians/comments/18bpb5/ive_always_heard_that_in_the_middle_ages_people/
Its pretty much a compete myth. To start with, any historical claim needs to have some sort of evidentiary basis. The claim that humans systematically struggled to find clean water has no basis in either the written or archeological record. What there is record of is lots of people drinking water. There are even written accounts of people in urban areas recognizing that their water is polluted, but instead of turning to beer as a savior they would simply build huge engineering projects to bring fresh water to them.
Undoubtedly, there would have been more water borne diseases in early times than there would be in current times via state of the art water treatment systems, but there is zero evidence of a widespread health emergency due to poor water that turned people towards beer.
Systemic bad water is mostly a (relatively) modern problem driven by overpopulation.
I'll leave someone to talk about beer, but fyi, you'll find a bit on water in the FAQ
In Ancient Egypt, beer was a source of calories as well as an intoxicant. Though what they made was more of a fermented porridge than a beverage.
And for what it's worth, the Egyptians themselves appear to have thought beer was healthier than water, or at least a common alternative to it. Worker's pay often included a beer ration several times a day and people from all strata of society drank it (including children). Frustratingly, I don't think I've ever read anything explaining why beer was so prevalent. Maybe there's nothing in the record, maybe I just have a blind spot on the subject. It's possible people just like to be drunk, but it seems unlikely that a society would devote so many of their available calories to something that was purely for fun--or that employers would offer intoxicants to their workers, people who were supposed to be getting shit done, if there wasn't a good reason to do so.
That the workers really, really liked beer is obviously a good reason in some circumstances, but so is a bias against water. And the water in Ancient Egypt might have been pretty disgusting, as all of it came from the Nile and the Nile was one gigantic, combined ecosystem with little input from external sources--rainfall is measured in mm in Egypt. In the lower kingdom, almost every drop of drinking water would have filtered through the cess pits, swamps and dung-filled fields of the lands upstream.
Unrelated to either your question or the rest of my answer, there's also some evidence that metabolizing alcohol is an evolutionary adaptation. Ground-dwelling primates (us) would have come across fallen, fermented fruit and if we could eat that fruit, we'd have a bonus source of calories. Thus, it's possible all alcohol contributed to the survival of all humankind.
Sources: Red Land, Black Land for the Egypt stuff, journals for the primate stuff. Such as: https://www.sciencenews.org/article/origins-alcohol-consumption-traced-ape-ancestor