According to the U.S. government website:
"The U.S. Passport Card can be used to enter the United States from Canada, Mexico, the Caribbean, and Bermuda at land border crossings or sea ports-of-entry and is more convenient and less expensive than a passport book. The passport card cannot be used for international travel by air."
So, even if may be permissable in some situations to use a passport card as your entry document, we recommend the full passport book for the following reason: in case of an emergency that required you to fly home from one of the ports, such a flight may be denied by customs. You should check with the cruiseline with which you plan to sail to learn their policies on this matter.
Here's a link to a page with further information on the U.S. Passport Card: http://travel.state.gov/content/passports/english/passports/information/card/Difference-Between-Passport-Book-and-Card.html