Typically you don't need your passports to get on and off the ship in the Caribbean. That may be different on Med cruises... I just put ours in the room safe after we board. The sign and sail card is all you need to board the ship in ports. (Bring an ID too. Some port security requires the ID along with the S&S card.
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