A closed-loop cruise is specifically defined as a voyage that departs from and returns to the exact same U.S. port (e.g., a round-trip from Miami to Miami). If your cruise begins in one U.S. city and ends in another (e.g., departs Fort Lauderdale but ends in San Juan), it is not closed-loop. This distinction is vital for U.S. citizens because, under the Western Hemisphere Travel Initiative (WHTI), closed-loop cruises allow for travel to select Caribbean, Mexican, and Canadian destinations without a passport. Instead, travelers can often use a government-issued birth certificate and a photo ID. However, even if a cruise is closed-loop, some specific countries (like Martinique or Guadeloupe) may still require a passport to disembark. Always verify your specific itinerary with your cruise line, as they may have stricter internal policies requiring a passport regardless of the legal "closed-loop" status.