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.
If that was supposed to be German for "cold clear water" that's "kaltes klares Wasser" and we don't capitalize adjectives.
Many sources on the net, but this one has the most nuanced discussion that I could find:
https://old.reddit.com/r/German/comments/53ws8q/are_ws_alway...
I only cosplay Deutscher Sprecher.
Most of my German linguistic skills come from listening to Chicks on Speed.