Are passports required?

It is recommended that all guests travel with a valid passport during their cruise. This will enable guests to fly from the U.S. to meet their ship at the first port should they miss their scheduled embarkation and allow guests that must disembark the ship before their cruise ends due to an emergency to fly back to the U.S without significant delays and complications. It is also recommended that all guests travel with a passport valid for at least 6 months beyond completion of travel.

I think you can also use your birth certificate or a passport card.