According to 2026 Reddit travel communities (like r/Cruise and r/immigration), the consensus is that while a Green Card (Permanent Resident Card) is powerful, it is rarely enough on its own for international cruising. For "Closed-Loop" cruises (trips that start and end in the same U.S. port), U.S. citizens can often use a birth certificate, but Green Card holders must carry their valid passport from their country of citizenship alongside the Green Card. Most cruise lines, including Royal Caribbean and Carnival, now strictly enforce this because if an emergency requires you to fly home from a foreign port (like Mexico or the Bahamas), you cannot board an international flight with just a Green Card. Reddit users frequently warn that while some Caribbean islands allow entry with just a Green Card for "day visits," the airline or cruise line may deny you boarding at the pier if you lack a passport, as they are responsible for your return to the U.S.