I carry everything in a clear backpack. With
school supplies starting to be in the stores, the clear backpacks are easy to find. They're waterproof and you can find your items quickly.
If you're concerned about people seeing your "stuff", just put a small makeup type bag inside the backpack and it hides it (but security will have you open any "hidden" bags).
Also, for my camera (very expensive digital), I carry it in a camera bag inside my backpack. That's double protection on wet rides, but the inner bag never got wet.
By doing the clear backpack, the only ride I "worried" about was the raft ride in AK, but it was fine. Just put your backpack in the center section and NOT at your feet!!!! Also, DON'T forget it when you get off.
I rode TOT and RNR and EVERYTHING with my backpack and had no problems. There's space at your feet for backpacks - you can hook your leg through a strap, tuck the backpack behind your legs, and don't worry. Mission Mars had a "pouch" for belongings.
Don't go "oversized" on the backpack, but a "standard" one is completely fine after you fill it with your stuff.
Oh, carry your backpack with you.
Good luck.