Yes, but only under very specific conditions known as the "Closed-Loop" exception. If you are a U.S. citizen sailing on an MSC cruise that departs from and returns to the same U.S. port (such as Miami, Port Canaveral, or New York) and stays within the Western Hemisphere (visiting places like the Bahamas, Bermuda, or the Caribbean), you can board using an original state-issued birth certificate and a valid government-issued photo ID (like a driver's license). However, MSC and all major cruise lines "strongly recommend" a passport. The reason is a safety one: if you have a medical emergency in a foreign port or miss the ship and need to fly back to the U.S., you cannot board an international flight without a passport. Birth certificates are only valid for sea travel, not air travel. If your MSC cruise is "One-Way" (e.g., Miami to Venice) or takes place entirely outside the U.S. (e.g., a Mediterranean or Red Sea cruise), a passport is strictly mandatory for all passengers, including infants, and you will be denied boarding at the terminal without one.