Contrary to popular belief, your premise is not true: drinking water in premodern times was not regularly unsafe. People knew how to avoid contaminating it and what water to avoid, and knew that boiling questionable water would render it safe. Alcoholic beverages were not regularly used as safer alternatives to water, because no safer alternative was necessary. This is a common topic on this sub; see, for example, this post answered by u/sunagainstgold, this answer by u/Qweniden, and this thread in which u/butter_milk both addressed the question directly and linked to additional posts asking the same thing.

As for your question about fetal alcohol syndrome, see this excellent answer by u/SpicyBaconator. The TL;DR is that the condition was first formally described in 1973.