Different mechanisms to move mass have different energy requirements. Shocking I know. Cars or land based transportation in general suffers from having a lot of mass since they tend to have higher rolling resistance. But once the mass is rolling, the propelling method can be turned off and that inertia is maintained. Remember, unless an external force is applied, an object that moves will maintain its velocity. It is also why trains are more efficient since they have very little rolling resistance.

Now, I am not very familiar with ships but I think it has something to do with the overall geometry of the ship itself that will define the energy needed to operate rather than mass. Just like airplanes, ships are moving in fluids so their geometry has a lot of effect on their engine efficiency rather than mass carried.