It is highly recommended that all cruise ship passengers travel with a valid passport book.
Although a passport may not be required for U.S. citizens taking cruises that begin and end in the same U.S. port, travelling with a passport enhances your disembarkation experience, as delays may be expected upon your return to the U.S. if you do not have one. A passport book will also enable passengers to fly from the U.S. to meet their ship at the first port should they miss their scheduled embarkation. Most importantly a passport book allows passengers that must debark the ship before their cruise ends to fly back to the U.S. without significant delays because of medical, family, personal or business emergencies, missing a ship’s departure from a port of call, involuntary disembarkation from a ship due to misconduct, or other reasons.
If they do not have a valid passport book, passengers who need to fly to the U.S. before their cruise ends will likely experience significant delays, expenses and complications related to the booking of airline tickets and immigration processing at the port of entry.
Passport cards can be used as a travel document for cruises but cannot be used when travelling by air, so a passport book is the best option in case a passenger must return home by air due to unforeseen circumstances.
U.S. Customs and Border Protection regulations on cruise ship passengers can be found here.