You can't accurately predict when and where hurricanes will form before the fact. To help you feel better, though, there are some things to remember. First, a hurricane of this category is extremely rare. Even if there is a hurricane out there when you cruise it is unlikely to be Katrina sized. Second, although there have been many instances of ships cruising while a tropical cyclone was active, there is such a good warning system once they form that the ships are able to maneuver around them. Sometimes this can cause a change of itinerary, but that can be fun, too. People have even been known to get an extra day or two on the ship! Remember there were ships out there even during Katrina and as far as I've heard they all made it back safe and sound (some to different ports, though).
Disney will keep you safe... relax and have a great time!