Budget Car Rental seems to be the company of choice, since they have a shuttle to and from Port Canaveral. National is also there, but you need to take a cab since they don't have a shuttle.
In case you should decide not to use a rental car to the Disney cruise ship, you could schedule a car service. It would run you about $200 round trip from Orlando International Airport. That is a far cry from the $70 per person round trip figure for the Disney bus transportation. A family of three would save money riding car service. A car service would also provide you with a grocery stop. You are able to take brew, water and liquor aboard the ship. There is an ABC liquor store about 3 miles south of the boat and a Publix grocery store about 4 miles south.