If you are a U.S. citizen, you can go on certain cruises without a passport, provided the voyage is a "Closed-Loop" cruise. A closed-loop cruise is defined as a trip that begins and ends at the same U.S. port (for example, departing Miami, visiting the Bahamas, and returning to Miami). For these specific itineraries, U.S. Customs and Border Protection (CBP) allows entry with a government-issued birth certificate (with a raised seal) and a valid government-issued photo ID, such as a driver's license. Children under 16 typically only need their birth certificate. However, there are significant risks: if you miss the ship in a foreign port or have a medical emergency that requires you to fly home, you cannot board an international flight without a passport. Additionally, some specific destinations (like Martinique or Guadeloupe) may require a passport for entry regardless of the cruise's "closed-loop" status. Therefore, while it is legally possible to cruise with just a birth certificate on many Caribbean, Mexican, and Alaskan routes, travel experts and cruise lines strongly recommend a passport to avoid being "stuck" in a foreign country during an emergency.