It's actually a song title, so properly capitalized, but I mussed the V v. W because I do not speak German and forgot that when she sings "VASSuh", it's spelled with a W. :)
Ok, as a German, I think we just don't care about the distinction. We only have the two letters/sounds W and F, the first representing various sounds between [w] and [v]. I think it isn't even a dialect thing, i.e. it is more like unspecified behaviour not like implementation-defined, it can change by time-of-day by a single person, because we just don't care. The letter V can represent either the German W or F sound, I think you just need to know that for every word.
Perhaps he has a familial history of same and is acclimated to drinking more water than you are comfortable with.
Perhaps he just enjoys drinking water. I do. As cold as possible. Kaltes Klares Vasser.
Regardless, ain't none o' yr dang bidness.