You've gotten some really good info on juice, so I dont have much to add about that.
I am not sure if you are bfing or not or if it would even make a differance, but my dr(s) have addvised me that my ds didn't need any additional liquids in his diet at this point and he is 10 mo. That the BM and the pureed food, fruit, veggies, had more than enough water. I give my ds water in the cup at meals because like others have said, it is more about getting familiar with drinking from a cup vs nutrition. My ds still has not mastered the art of cup drinking but enjoys the chance to play with the water. He probably only manages to get a couple good sipps at best. But that bring me to my question, why give anything besides water?