Thanks. This explain why cruise ships are much more wasteful than ferries:

perhaps the biggest difference is the payload. […] The reason, of course, is that a cruise ship is a floating village, with shops, restaurants, swimming pools and the like

This is why it’s important to compare transportation mode all things being equal. Keep in mind there’s lot of people flying coach, very few planes with individual cabins and restaurants. Cruise ships are designed for entertainment and luxury. Ferries are designed for transportation.