Cruise Fanatic has it correct. We always take a change of clothes in a carry on because you don't know how long it will be before you get your luggage. And sometimes you get most of it, but the bag you really need comes late...
Better to be safe than sorry. And if you are flying on the day of the cruise, there is no guarantee the airline was lose a bag. They will eventually get it to the ship, but if you don't have a change of clothes, you first day can be ruined.
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