"I want a glass of water."
Translation:Tá gloine uisce uaim.
You never say gloine de fíon. For one thing, de lenites, so it would have to be gloine d'fhíon, but you're confusing the "partative of" ("a slice of cake", where "of" indicates the relationship of a part to the whole), with the "of" in "a glass of wine"
This "partitive of" can be expressed with de (the partitive dative) with the "of" in "a glass of wine", which just provides us with information about the contents of a container. You can use de for the "partative dative" - píosa de cháca, duine de na fir, and in some other uses of "of", but for many other uses of "of" the genitive is more appropriate.