"You are drinking water."
Well, that's the 'fault' of Polish case system. I see you're rather at the beginning of your adventure with Polish, you will soon come around many comments that ask similar questions and Polish people answering them in details :)
In this particular example: Pijesz kogo? co? = you drink whom? what? (forget that it doesn't make any sense to 'drink someone' --> Accusative. Thus, "wodę'.
A funny thing about English is this sentence can also mean 'jesteś wodą do picia'.