Best answer - maybe. It depends on the cruise itinerary. BUT, if there's an issue and if this other guest needed to fly home from a foreign port, a card doesn't work.
On a closed-loop cruise that leaves from and returns to the same US port, a passport card will work (as will a US birth certificate). However, as PrincessShmoo said, the card will not be sufficient if someone needs to fly back to the US from another country.
I agree with what others have said. You could use it as a valid form of ID, but if something goes horribly wrong and you need to fly home from a place outside the US (with the possible exceptions of Canada and Mexico), the passport card will not allow you to board a plane.
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