The speed of the ship makes a huge difference in fuel consumption. Cruise ships/ocean liners usually go as fast as practicable, which leads to terrible fuel efficiency. Reducing speed from 20 knots to 15 knots may cut fuel consumption in half or more.

One thing I’ve noticed is that a lot of comparisons you’ll see will have, for example, a 5-day journey by cruise ship compared with an airline flight covering the same distance. Usually measured not in fuel consumed but carbon emissions. Ok, but shouldn’t the more accurate comparison be a 5-day cruise compared to an airplane trip plus 5 days in a hotel?