If your passport does not arrive in time, your ability to board the cruise depends on the ship's itinerary. For "Closed-Loop" cruises (voyages that begin and end at the same U.S. port and stay within the Western Hemisphere), U.S. citizens can technically board using an original birth certificate and a government-issued photo ID. However, this is risky; if you have an emergency and need to fly home from a foreign port, you cannot board a plane without a passport. If your cruise is "Open-Loop" (ending in a different port) or visits destinations outside the Western Hemisphere, you must have a valid passport to board; without it, you will be denied boarding at the pier with no refund. If you realize your passport won't arrive, your best option is to secure an "Urgent Travel" appointment at a regional passport agency, which can often print a passport in 24-48 hours. Always check your specific cruise line’s documentation requirements, as some luxury lines mandate passports regardless of the route.