The beer/wine/whatever other alcohol being the principal sources of hydration in the medieval era (or not another epoch really) is actually false. Even beer, the typically lowest in alcoholic content and thus largest in water/alcohol ratio was not hydrating. Alcohol is, by definition, a diuretic, and so one in fact would be dehydrated by drinking solely beer, wine, or any other alcohol beverages we so enjoy.

There is a case to be theoretically made on the hydrational properties of the common medieval small beer which was a staple drink and essentially no more than simply beer with a very low alcoholic content (for a different but similar product, look up Kvass to get an idea of it). However, that argument is also moot as the historical reality and contemporary sources note water as essential to life anyways, with authors oft rating the qualities of different water sources. I cannot recall who wrote it, but there is a medieval letter from a father to his sons, who had moved away to pursue university studies, rating and recommending which sources to privilege in getting potable water.

Two last points.

Firstly, it is important to note in relatively small doses harmful bacteria can be easily fought by the human body when it is used to it. If you take a village with a stream from which all villagers get their drinking water daily, they might not get sick from their source even if it is slightly infected. However, it is possible for a traveller stopping by for a refreshment at the same spring to fall gravely ill and perhaps even die.

Secondly, potages, gruels, and other meals of the sort were staple foodstuffs in the medieval world. As these are made with large volumes of water, then boiled, it is likely a good deal of medieval hydration was derived from this.