For international travel, U.S. citizens need to have a passport or other valid documents in order to enter the United States. Caribbean cruises that begin and end in the U.S., called closed loop cruises, do not require you to travel with a passport. However, if you need to leave the cruise ship because of an emergency and you do not have a valid passport, you may have trouble entering or staying in a foreign country. You may also have difficulty re-entering the United States by air because many airlines require a valid passport in order to board the aircraft. Because of these reason, it is strongly recommended that you always travel abroad with a valid passport.