Pat answered, and I copied this from wdwforgrownups:
It's
important to remember that if you are traveling abroad from your home you are subject to the laws and customs of the country you are visiting. Contrary to popular belief, the
Disney Cruise Line is not a little piece of America at sea. The
Disney Cruise Line ships are flagged in the Bahamas, while on board you will be subject to Bahamian and International Maritime Law, as well as those laws of the country where you are in port.
Do I Need A Passport For A Disney Cruise?
For
U.S. citizens a passport is not required for travel on closed loop cruises (a closed loop cruise is one that that both departs from and returns to a U.S. port).
Exact identification requirements for all travelers sailing onboard the Disney Cruise Line can be found at the
Passport & Documentation Page on the DCL website. However,
if any sort of emergency were to occur while overseas, you would not be allowed to fly home without one. That is true whether the unlikely event is one where the ship is at fault, or a personal or medical emergency. All
air travel into the U.S. from foreign countries
requires a valid book-style passport. Therefore, both the Disney Cruise Line and I highly recommend that guests of all ages traveling onboard DCL have a valid passport to offer an extra level of security while visiting overseas.
Passport Cards may be used as identification cards. However, like any other state issued ID, they
can not be used to fly home in an emergency from any foreign port. Therefore, optimum choice is to have a book-style passport book.