You are not sure of your nationality?
Here is what I've seen done:
I've seen people put their birth certificate number (if it has one) in the passport blocks. Otherwise, I'd just leave it blank.
Port of embarkation is where you boarded the cruise. Miami, Port Everglades, etc.
Nationality is the country you are a citizen of....US, Canada, UK.
Intended address in bahamas: the ship's name.
The Bahamas is a place that prides itself on archaic bureaucracy, and these cards are a vestige of that. I seriously doubt they spend a lot of time carefully going over each card before letting passengers leave the ship. The tourist dollars outweigh it! These type of cards used to be more common but most other islands have stopped using them.