I wouldn't do it without getting reservations first. You can probably get a shuttle at the "taxi/limo" stand without one, but why risk it? We are doing one way car rentals. You pick the rental up at the airport and drop it off in Port Canavreal. The rental place has a constant shuttle to and from the port. The cost of a minvan one way was less than half of what the cruise line was going to charge for their service. Other options are pre-arranged limo/shuttles if you don't want to deal with the rental. But I would not do it without a reservation.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007