Its partitive-ness, if you will, is emphasized here, so that declension supersedes the accusative--she is not asking for any specific water, or that would be accusative (воду). She is just asking for "some water" (without regard to a specific kind), making the partitive ending more important (воды).
At first I thought that partitive wasn't a case on it's own, doing some research proved me wrong. Partitive is/was a case in Russian, however it has been merged with the genitive case. The partitive case is more historic grammar. According to the book "Case, Valency and Transitivity" the Russian case system was influenced by the Finno-Ugric language group. Which also have case systems.