"How much water do you drink?"
Translation:Quanta acqua bevi?
Use the article if you are talking about a general category: "I drink water" could be "bevo acqua" or "bevo l'acqua." You drink it or you don't, but it is a general statement that applies to all water.
But when you are asking how much water, it indicates you are drinking some but not all the water, so you don't use the article. You're only talking about the specific amount of water that one specific person drinks in one day.
Also, although both the English "drink" and "am drinking" would both be "bevo," if you said, "I am drinking water," you usually aren't making a general statement about whether or not you drink water, so no article.
I think "quanta acqua bevi tu" would be correct instead (although I did not try it). In Italian the subject (tu) is usually placed at the end of the sentence in questions where there is an interrogative word (quanta) at the beginning. See: http://italian.about.com/library/fare/blfare115a.htm
Select which word to use according to the gender of the following noun and whether it is singular or plural:
Quanto - masculine singular, e.g. quanto pane (How much bread)
Quanta - feminine singular, e.g. quanta torta (How much cake)
Quanti - masculine plural, e.g. quanti ragazzi (How many boys)
Quante - feminine plural, e.g. quante ragazze (How many girls)