Pport for Caribbean cruise?

Have not seen those before...but... What about if the cruise ship stops at a foreign country and you want to get off to visit...? Also, even though the WH TI does not yet require pports for sea travel, some cruise lines do require their pengers to have pports. I figure this is for ease of I.D., and in case of emergency...if the ship has to stop in an unexpected port or if a penger needs to be put off at an unexpected port. You could ask one of the cruise lines...they should be experts on this and could explain their way of thinking.