Looks like you have to get it done before the cruise according to their website.
TESTING Requirements
Fully Vaccinated Guests (2 shots or 1 J&J shot): Must provide negative Covid-19 test results, that is no more than two (2) days old, prior to embarkation.
Up to Date Vaccinated Guests (2 shots or 1 J&J shot + booster): Must provide negative Covid-19 test results, that is no more than three (3) days old, prior to embarkation.
Unvaccinated Guests must provide a PCR negative test result, taken not more than 2-days prior to embarkation and be tested at the port prior to embarkation.
Unvaccinated Guests under 2 years old have no Vaccination or Testing requirements.
Cruise and Stay and One-Way Guests: All vaccination and testing requirements apply both during exit and re-entry into the US. In addition, these guests must provide a copy of the Bahamas Travel Health Visa before boarding.