Like "Luanne" said ... normally passports are not taken from US citizens on CARIBBEAN cruises. The only times the ship has taken ours was on European, etc cruises.
What we did on a Caribbean cruise in January was make a copy of our passports to take with us on shore excursions. We were in quite a few ports (14 days) . We (and everyone else) actually had to show the photo ID in two ports before reboarding the ship -- Key West and Grand Cayman. The COPY of the passport worked fine.
PARENTS: They only wanted photo ID's from adults. Maybe this will help someone.