I bought 2 to try when they were on sale, but they weren't for me, or at least the ones I bought. Both were too heavy. You may have better luck. I advise trying them out in advance.
Another thing we try to do is carry as little as possible. that is probably another reason I didn't bother with them. The clothes are at least light/easy to carry.
A heavy bag on your back is like wearing a jacket. That said, some water is essential. We will also pack a few tangerines in our bag, but then eat them by 10am, so we aren't carrying it around very long. (WDW sell tangerines)
You can get free ice water at the quick serves, and Universal has fee water at the Coke Freestyle machines, but asking for water all day gets to be a bit much. A hybrid approach is better, of bringing some with you, then getting more as the day goes on.
Worth noting, some water tastes better than other places.