You can use a birth certificate for a Carnival cruise only if the voyage is a "Closed-Loop" cruise, meaning it begins and ends at the same U.S. port (e.g., Miami to Miami) and travels within the Western Hemisphere (Mexico, Canada, Caribbean, Bahamas). In 2026, U.S. citizens must present a state-issued, original or certified copy of their birth certificate (not a photocopy or a hospital-issued "souvenir" certificate) along with a government-issued photo ID, such as a driver's license. However, there are significant risks: 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. Additionally, certain destinations like Martinique or Guadeloupe may require a passport even for cruise passengers. While the birth certificate is a legal "minimum" for entry back into the U.S. by sea under the Western Hemisphere Travel Initiative (WHTI), Carnival and travel experts strongly recommend a passport for all international travel to avoid being stranded in the event of an unforeseen itinerary change.