It's always nice to travel with a passport, required or not, it is the preferred most recognized source of identification in a foreign country. Cruise lines will not book you on a European cruise without a valid passport, due to each foreign country entry regulations for visitors.
However the rule for cruises to the Bahamas, Caribbean, Canada, and South America, is as long as the ship embarks from a US port, and the cruise ends with disembarkation in a US port, referred to as a closed-loop. You should have no problem going ashore, exiting and re-boarding the ship in any port of call with just your ships identification-key card.
However the rule for cruises to the Bahamas, Caribbean, Canada, and South America, is as long as the ship embarks from a US port, and the cruise ends with disembarkation in a US port, referred to as a closed-loop. You should have no problem going ashore, exiting and re-boarding the ship in any port of call with just your ships identification-key card.