If the ship begins and ends its voyage at the same domestic port, passengers can board by showing an official government birth certificate and — for anyone 16 or older — a government-issued photo ID.
Of course, there are some exceptions: Barbados, Guadeloupe, Haiti, Martinique, St. Barts, St. Martin, and Trinidad and Tobago all require cruise passengers to have a valid passport in order to enter their countries. However, you can sail on a cruise that calls on these islands, as long as you stay on the ship at these ports.
One point to remember: Just because you can cruise without a passport doesn’t mean you should. If you were to miss the ship, not having a passport would make reentry into the U.S. a nightmare. If it’s just a few weeks before your trip, consider taking advantage of the government’s passport rush service or a private service like rushmypassport.com for less hassle.