It seems pretty clear now, that quarantining on a cruise ship doesn't work, for whatever reason. In fact in this case it appears to have simply been a waste of time, as passengers now have to undergo quarantine again ashore.
The long incubation time, now thought to be more than 14 days, makes matters very difficult - an entire cruise could take place, with an infections person amongst the thousands of passengers, and also when you add on that one of the most vulnerable groups is the elderly, its just not good all round.