Technically, for U.S. citizens on a "closed-loop" cruise (one that starts and ends at the same U.S. port), you are not legally required to have a passport under the Western Hemisphere Travel Initiative (WHTI). In 2026, you can still board with a government-issued photo ID and an original/certified birth certificate. However, this is highly discouraged by travel experts and cruise lines. If you encounter a medical emergency or a mechanical issue in the Dominican Republic and need to fly home, you cannot board an international flight back to the U.S. without a valid passport book. Some specific cruise lines or premium itineraries visiting the DR may also mandate a passport for all guests regardless of the closed-loop rule to ensure smoother customs processing. Furthermore, a Passport Card is acceptable for the cruise itself but, like the birth certificate, will not allow you to fly home in an emergency. For the safest experience in 2026, a standard passport book is the gold standard.