I actually brought my 5DII and 100-400 with me into Animal Kingdom Park on three different occasions, and I got awesome shots of the animals. All I brought was the camera and lens hanging on my Black Rapid strap. Oh, and a Shorty 40 in my pocket. No water rides. I was able to hand-hold this kit on the big roller coaster at the park without a problem.
I brought my 5DII and 24-105 in a couple other parks, and hand-held it on thrill rides, but toward the end of the trip, I was only bringing my S95 point-and-shoot. The weight and hassle wore me down.
I would put a UV/clear filters on the lenses you bring. There will be moisture and I prefer to know my lenses are covered. The 24-105 is a good because it has weather sealing, but you need a filter on the front element. I would also bring dessicant packs, as you will be going in and out of A/C and heat and lots of humidity. To protect your camera, you could bring a big ziploc and put your camera in it during the water rides. Keep it simple.
EDIT:
Check out my Flickr page if you'd like to see some shots taken during my trip to Disney World.