First trip with our ds5 was when he was 4 months old. We switched to powdered formula to lighten our load. We just added cold fountain water--no problem. Except he got the runs. After his next trip to WDW (and the runs) we realized it was the FL water. First I wouldn't let him drink the park water. We have a relative who lives in Kisssimmee with her water bottle and he had some of that. No good. We buy a load of water at Publix, freeze it and bring it or we buy it. Nothing's worse than having a child with the runs in WDW if you can prevent it.
If you have a sensitive stomach, don't drink it. But if you just don't like the taste, I think the packets are the way to go.
I saw this family bring in this HUGE cooler full of water and snacks (maybe it was 2 or more families together) and they rented a double stroller to wheel it around.