I know the documentation required and processing time, as well as the length of validity are the same for the Passport Card and the book Passport; they have the same objective: identifying the holder as a U.S. Citizen.
But the Passport Card was developed as a convenience to those who make frequent land crossings between the United States and Canada and Mexico. Other than that it is no more useful than a certified birth certificate and photo I.D.
And to answer your question; you don't need your passport or passport card to go ashore in Ocho Rios or Grand Cayman--just your cruise card and photo I.D.
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