I was never a boyscout, but wherever I go, I always go prepared. Everytime I go to Disney or any other amusement park (not that WDW is able to be classified as "just an amusement park"

) I carry a full pack with everything we might need. I usually carry a full load of water (as much as is comfortable to carry), some snacks for the day (stored next to the frozen water bottles to keep cool), towel, extra shirt, camera(s) and sunglasses.
Even with all that crap, I've still never had trouble taking my pack with me on a ride. I usually just put it on the floor between my legs, and put my legs through the straps where you would usually put your arms. Even on rides that go upside-down, I've never lost a bag (but I have had to squeeze my legs to hold it in place on a few rollercoasters!) I've also discovered that most backpacks are naturally water-resistant, so unless they are sitting in a puddle of water for more than 10 minutes, nothing inside will get wet. I've taken my bag on Kali, and everyone with me put our shoes inside--everything came out dry.
I would not reccommend carrying such a big pack with you all day, since it can even wear me out after a full day in the Florida sun, but I wouldn't be afraid to take a good, sturdy bag anywhere.