While the Cayman Islands do not require US citizens to present a passport, Americans need a passport or passport card to re-enter the United States. The only exception is for US citizens who are on a closed-loop cruise (one that begins and ends at a US port).

The answer depends entirely on your cruise itinerary. If you are on a "Closed-Loop" cruise (one that begins and ends at the same U.S. port) that stops in Grand Cayman, a U.S. Passport Card is technically acceptable for re-entry into the United States under the Western Hemisphere Travel Initiative (WHTI). However, the Cayman Islands government and the cruise lines themselves strongly recommend a Full Passport Book. The critical risk of using a Passport Card is that it is not valid for international air travel. If you were to experience a medical emergency, miss the ship, or have a family crisis that required you to fly back to the U.S. from Grand Cayman, the Passport Card would not allow you to board a plane. Most cruise lines like Carnival or Royal Caribbean will allow you to board with a card, but they will warn you that you are essentially "trapped" on the ship or island if you cannot complete the journey by sea. For total peace of mind, the book is the gold standard for all Caribbean travel.