I know it’s impossible to know exactly and people weren’t out boozing all day long. But particularly with wealthy people back then having wine and such with meals.
No, because you're starting from a false premise here. Yes, the Medievals had excellent access to fresh water sources. Indeed, the first and most important consideration when founding a village was where to get safe drinking water. The thought of the Medievals drinking alcohol all day is a myth which it is my life's work to kill (hence my flair!); see further here.
On the matter of fetal alcohol syndrome,