If you are a US citizen, a passport for that cruise is not needed.
If, however, while in Nassau you find that someone has an emergency and you must fly home from there, you will need a passport. Or if someone is admitted to the hospital you'll need a passport to be there and to get home. (which means hours in a gov't office...you won't be stranded, but you'll have to get it done) Or if you're on
Castaway Cay and something horrid happens and you're airlifted to a nearby island, it'll be the same. A passport will be needed to get home by plane.
It's a pretty low chance for such a short cruise, but it's all about YOUR level of risk, and YOUR level of irritation at having to sit in a gov't office paying way more than you would have paid for a normal passport (and I hear that the emergency passports have a much shorter expiration than the normal ones).
It also depends on how likely or hopeful you are that more international travel will come up.
For instance, I hear that trips to England are a VERY good value right now. I saw a deal for roundtrip tickets to Europe starting at $576, for instance! If you have a passport, taking advantage of that sort of thing is possible.
