Extract a sugary liquid from fruit or malted grain, expose it to the pervasive and random yeast in the air around us and if it doesn't mold over first you'll probably get something in the 8% ABV range before the alcohol waste product inhibits the yeasts' metabolism. With some deliberate cultivation, which humans thousands of years ago figured out, you can selectively breed the yeast slurry in your vessels up to a 11-14% alcohol tolerance. They didn't know the science but they knew the process.
Leave it exposed and you'll eventually get a random acetobacter infection and ta-da, now you have a vat of vinegar. Which would still have utility, but is much less fun.
The 1-2% alcohol drinks were called "small beers" and were the second runnings off the grain that was used to make full-strength beers or barleywines. Humans liked variety.
And people drank water, not just alcoholic small beers, there are depictions of water filters dating back at least 3500 years, and a long history of water purification techniques, and a ton of ancient wells which were dug.