I have never taken a camera bag into the parks with me.
However, I do always take a
Camera Poncho (basically a 1-gallon Ziploc bag) to protec the camera from rain or water rides.
You mentioned "an" extra CF card. I recommend you hurry up and find some inexpensive cards and buy more before your trip! Unless you have a laptop to download your pics nightly, you should have 1 card for at least every two days of trip time, and ideally one card for each day, so that your pics don't all wind up on the same card. One false move, one card or camera malfunction, or one good swan dive from a POTC boat, and you could lose your whole trips worth of pics, unless you divide them up onto multiple cards.
Enjoy! Your pics will be the best you've ever taken at WDW.