Technically, you cannot go on a cruise without any papers, but you can often cruise without a passport if you are a U.S. citizen on a "Closed-Loop" cruise. These are voyages that begin and end at the same U.S. port (e.g., Miami to Miami) and travel only within the Western Hemisphere. For these trips, an original government-issued birth certificate and a valid driver's license (for those over 16) are usually sufficient for boarding. However, cruise lines and travel experts strongly advise against this; if you have a medical emergency or miss the ship in a foreign port, you cannot fly back to the U.S. without a valid passport. For all other international cruises, a passport valid for at least six months beyond your travel date is a non-negotiable requirement.