Castaway is the Bahamas. have an emergency there you might be flown to Nassau vs Florida. Then you have to get home from there. Which will need a passport book. Which you can get there, expedited and expensive and sitting in an office instead of being on a plane, but you won't be stuck there. It's just going to be expensive and annoying.
BUT that's very unlikely to happen. The Card seems to be fine for the cruise. I mean, we have the Books. I have no interest in expedited passports and being annoyed in offices. But I do try to be realistic.