Comparing passenger jet emissions to cruise emissions is missing the point. The point of a jet is to get from point A to point B ASAP. The point of a cruise ship is to have a nice time, and maybe get from point A to point B. AFAIK many (most?) cruise routes even go back to the port that they depart from!
On a nice hotel, because most cruises are nice hotels that sail. The amenities you can have in a cruise are far, far beyond what you would get on a first-class international flight.
