"Do you have water?"
Translation:У вас есть вода?
If I'm not mistaken, ты is the familiar singular form of "you" and вы is every other form of "you" (i.e., plural familiar, singular formal & plural formal). Тебя and вас are used for both the accusative case (direct object) and the genetive case (possessor). If you're familiar with French or German, ты is like tu or du and вы is like vous or Sie.
The possessor needs to be in the genitive case so it would be "у тебя есть?" which should be one of the accepted answers.
I thought so too, but you must select ALL applicable answers, there are two. Hope this helped.