<ahem> How many times do I have to mention the closed loop exemption? I didn't fabricate it and make up a fake website to detail it. Google "close loop cruises".
So let me restate:
For U.S. citizens or legal residents, passage on a closed loop cruise does not require the person to have a passport. Look here
U.S. citizens require a passport to return to the U.S. via air or land - or if on a cruise that does not meet the requirements of the closed loop ruling. But for closed loop voyages if they leave and then re-enter the U.S. from the cruise ship, a passport is not needed. A photo ID and proof of citizenship (certified birth certificate) is still accepted.
I've never been asked for a passport in the Caribbean, either by port security or ship's security.
As I said earlier, it is strongly advised that everyone obtain and maintain a passport for all travel.
Since this topic has been answered (again), I'll now close it.