For U.S. citizens, you can take your son on a cruise without a passport only if it is a "Closed-Loop" cruise, meaning the ship departs from and returns to the same U.S. port (e.g., Miami to Miami) and travels within the Western Hemisphere (Canada, Mexico, Caribbean, Bahamas). In this specific scenario, a child under the age of 16 can travel using their original or certified copy of a birth certificate. However, this is strongly discouraged by travel experts and the U.S. State Department. If your son becomes ill, misses the ship, or if the ship has an emergency and needs to dock in a foreign port, he cannot board an international flight back to the United States without a valid passport book. This could leave you stranded in a foreign country for days while you seek emergency documentation from a U.S. embassy. Furthermore, some specific ports or shore excursions may still require a passport for entry. For the highest level of safety and to ensure you are prepared for any "worst-case" scenario at sea, obtaining a passport for your child is considered the gold standard for 2026 travel.