You can use a birth certificate for a cruise only if it is a "Closed-Loop" cruise, which is defined as a voyage that begins and ends at the same United States port and stays within the Western Hemisphere. Under the Western Hemisphere Travel Initiative (WHTI), U.S. citizens on these specific routes can present a state-issued, certified birth certificate (not a "hospital" certificate with footprints) paired with a government-issued photo ID, such as a driver's license. However, if the cruise starts in one city and ends in another (e.g., Miami to San Juan), a passport is mandatory. In 2026, most cruise lines—including Carnival, Royal Caribbean, and Norwegian—"strongly recommend" a passport even for closed-loop sailings. This is because if you have a medical emergency or miss the ship in a foreign port, you cannot fly back to the U.S. without a valid passport, as air travel regulations are much stricter than sea travel rules. Relying solely on a birth certificate can leave you legally stranded in a foreign country if your travel plans deviate from the original ship itinerary for any reason.