I don't believe the water is what makes the difference. It's the 'the'.
"I am drinking milk" is a different sentence than
"I am drinking the milk."
A 'de' would be necessary in the first, but not the second.
I think translating it into "Vous buvez de l'eau." is wrong because that would mean something more like "You drink from the water." Might not seem like much of a difference in English, but in French (and Spanish) the meanings are quite different.
Du (the contraction of de le) and de la are for unspecified quantities of something. "You drink water" or "you drink some water" would require de l'eau. Le / la are the definite articles, for saying "you drink THE water."