cruise ships

Question

Family of a friend from th eoffice told me that his family had taken a cruise, leaving port on monday and were supposed to arrive on friday morning. Due to a strom they were delayed and arrived at the port [of miami] after shabbat had started. They did not know what to do, since some thought it was forbidden to get off and now it was shabbat. Could you please explain exactly what is the problem of getting off a boat on shabbat is ? thank you. If you hold it is allowed,could you explain why other sforbid it and why you do not ? thank you.