For U.S. citizens, the answer is yes, but only for "Closed-Loop" cruises. A closed-loop cruise is one that begins and ends at the same U.S. port (for example, departing Port Canaveral, visiting the Bahamas, and returning to Port Canaveral). In these specific cases, you can board with a government-issued birth certificate and a valid photo ID (like a driver's license). Children under 16 only need their birth certificate. However, this is strongly discouraged by both Disney and the U.S. State Department. If you have an emergency—such as a medical evacuation or the ship being forced to dock in a different country due to weather—you cannot fly back to the U.S. from a foreign port without a passport. Additionally, some specific ports of call (like certain islands in the Eastern Caribbean) may have their own laws requiring a passport for entry even if the ship doesn't. To avoid being "stuck" in a foreign country during an emergency, it is highly recommended to have a valid passport book for every member of your party.