For U.S. citizens, you can go on a "Closed-Loop" cruise without a passport, provided the ship begins and ends its journey at the same U.S. port. These itineraries typically cover destinations in the Caribbean, the Bahamas, Bermuda, and Mexico. Instead of a passport, you are allowed to travel with a "Western Hemisphere Travel Initiative" (WHTI) compliant document, which most commonly means a certified birth certificate and a government-issued photo ID (like a driver's license). This also applies to cruises to Alaska (if they start and end in a U.S. city like Seattle) and New England/Canada cruises with the same start and end point. However, there is a major catch: if you are forced to leave the ship early due to an emergency, or if you miss the boat at a foreign port, you cannot fly back to the U.S. without a passport. Furthermore, some specific islands like Martinique or Guadeloupe may require a passport regardless of the cruise type. Because of these risks, both the cruise lines and the State Department strongly recommend a passport even for closed-loop sailings to ensure you aren't stranded in a foreign country without the proper legal documentation to fly home.