There was a post earlier about the cruise line 'siezing' your passport. At first I had a difficult time with this as it says right on your passport to never give it to anyone else. However, the reason that they do this benefits everyone.
Since the ports are international, when the ship docks, Passport officers are handed the passports of all the non-us Citizens on board. All passengers are then PRE-cleared, saving us ALL a huge hassle in the middle of our vacation.
Knowing THAT makes it much easier to take.
Hope that adds some insight