What Carol said and most countries that require a passport want it to be valid for 3 to 6 months beyond your arrival date. Although since this is new to the Caribbean, whos knows what will go into effect.
And US Customs may get finicky (you never know with Customs folks) toward the last couple of weeks of the year, they may decide they want you to have a passport if you are coming back during the transition period.
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, Celebrity Silhouette 2013