There seems to be two quite wide-spread opinions everywhere on the internet and both simply can’t be true: that medieval people drank water and that they didn’t.
E.g. arguments for drinkng water:
"Misconceptions About the Middle Ages, by Stephen Harris and Bryon L. Grigsby, says: “The myth of constant beer drinking is also false; water was available to drink in many forms (rivers, rain water, melted snow) and was often used to dilute wine.”
and arguments against:
http://www.sca.org.au/st_florians/university/library/articles-howtos/what-people-drank.htm
I would say neither of those sources is particularly reliable or well-founded. Is there some reliable authority we could consult?