Unless your husband is getting the passport card in place of a REAL ID drivers license/state ID card or is planning to be on a 1-way or Alaskan cruise, the card doesn't have much benefit on a cruise compared to a drivers' license and birth certificate. It would work on most cruises leaving from the US (though I haven't actually used one) but he may want to take other trips over the next 10 years.
A passport card can't be used for flights in and out of the country, so if something horrible happens during a cruise and he needs to fly (or be flown) home, he may need to get a passport book.
The card is valid for land and sea travel in Canada, Mexico, and the Caribbean, so it would work for Alaskan and Caribbean cruises but not for, for example, Europe or a Panama Canal cruise. Most Caribbean cruises start and leave from the same port, in which case a passport isn't required at all.
A passport card is easier to carry than a passport book, but that means it's also easier to misplace. It costs less, but if someone is going through the process to get a passport for a cruise (which costs thousands of dollars) spending $100 or so more to get a passport book would make more sense.