If you were unable to complete the cruise, such as sickness or injury, you would need a passport to fly home.
As a US citizen, to the Bahamas, I used my birth certificate and driver license. Didnt have a passport. But next time I will be using one since I I didnt liked having legal documents on the boat, althought there was a safe in the room.
Passport is easier to hold onto, but as long as you have your birth certificate, you should be ok.
We went to the Bahamas and only needed our birth certificates.
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