It was debunked time and time again. People in ancient times drank water, it's just that reading ancient texts gave some historians the idea that people only drank beer, since that was the only drink they wrote about(well, along with wine). No one wrote about drinking water because there was no reason to write about it. But modern historians are very much certain that the "they only drank beer because it was safer" thing is a myth, we don't have any proof for that.