You are doing well to notice that and think that through. So many others don't.
They really would NOT let you fly in an emergency without having a passport book. So instead of getting on a plane and going, you would be sitting in a bureaucrat's office, paying expedited fees, to get an emergency, instant, passport book.
If you want it "just in case", get the book.
If you want it just for ID, well, honestly, there's no strong reason since you don't travel anywhere else outside of the country. BC and ID will work just as well as the card, for the closed-loop cruise purposes.