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.
Check out my Flickr page if you'd like to see some shots taken during my trip to Disney World.