We always go through lots of water if we go to WDW in the summer!
Also, well, when people get dehydrated, their energy level drops quickly. But your water needs really depend how heavily you personally sweat.
We tend to load up in the morning, drink some water early, recycle a few bottles, refill others, get ice water with every meal, etc. In the past, asking for water could be a bit slow. On our most recent trip though, the CM's did a much better job of having some handy for guests. Still, most CM's will give you a somewhat small cup when you ask for water.
I DO wish WDW had more bottled water refill stations!