No, "wodę" is Accusative. It is used for the direct object of the sentence.
Nominative is mostly used for the subject of the sentence. "dziewczynka" here is Nominative.
In the first part of the course, the only Instrumental you will find will be in sentences like "She is a girl" = "Ona jest dziewczynką". Then other usages will come, most commonly after "z" = "with".