Hi livijean.. welcome to Cruise Chat!
Dave posted an FAQ response on passport rules here.
You are getting different responses from the government because the rules are so confusing and really depend on HOW you travel. Its really best to avoid problems and get the passport. You still have time to do so for a December cruise. While technically you don't need one for a cruise, what happens if something goes wrong during the cruise and you have to fly home (say you miss the ship?) Or even if you were to miss the ship the first day and have to fly to catch up with it in the first port? Without a passport you can't do this...
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