My understanding is that the passport card would be sufficient for the cruise and that passport books are only required for flights back into the US. And, of course, closed-loop cruises don't required any passport at all at the moment. I'm sure someone will correct me if I'm wrong. However, anytime you leave the country by ship, you run the risk of possibly having to return by air. Best to have the passport book. Naturally, there will be things you can do to get home if you're stuck in another country without a passport, but you will undoubtedly have to jump through a million hoops in the process. Who wants to do that when you're already stressed about missing the ship, and possibly a sick family member and maybe dealing with people with strong accents whose first language isn't English? This is one less potential headache.
Rhapsody of the Seas 12/2005
Carnival Conquest 11/2008
Carnival Holiday 10/2009