I used a Camelbak backpack for my June 2011 trip, and it worked great! Instead of filling it with water, I just filled it with ice from the machine at the resort prior to heading to the parks. As the ice melted, it kept my back cool and provided me with drinking water throughout the day. Since it was so hot and muggy out, my back would have been soaked from sweating anyway, so I didn't notice if my back getting wet was from the melting ice in the Camelbak bladder or from sweat. That's one of the hazards of wearing a backpack to WDW in the summer, lol, but you'll see everyone gets it, so it doesn't really make much of a difference. You can get cups of ice water for free from QS locations, so if you want to refill the Camelbak with that instead of the lukewarm drinking fountain water, you also have that option, too. Just be sure to take the Camelbak bladder out of the backpack to air dry at night. I'm not sure how long your stay is, but you don't want it to start to mildew.