"Do you want the water?"
Translation:Veux-tu l'eau ?
"du" is the contraction of "de-le" which cannot be used with "eau" which is a feminine word.
By the way, the English is about THE water, not (some) water.
it is perfectly correct and formal.
the two other forms are:
standard: est-ce que tu veux l'eau ?
casual/oral: tu veux l'eau ?