Power to overcome form drag goes up with the cube of the velocity and linearly with fluid density. Moving something through water requires over 800 times more power than in air.

The only thing that could make land-based transport less efficient (other than fuel and power-plant issues) is rolling and drivetrain losses. I doubt that those make-up for the over 800-to-1 power discrepancy.

The greater point here isn't a pissing match between individual efficiencies but rather to look at the problem with a long (50 to 200 years) perspective in terms of pollution, fossil fuel consumption, geo-political and economic concerns and more.

The drag of ships moving at less than hull speed (non planing) is a function of: skin friction (shearing of the boundary layer), wave making, and Reynolds number (related to the length of the ship, and the viscosity of the fluid).

One interesting article says that one of the mega container ships pollutes more than 50 million cars see:

http://www.guardian.co.uk/environment/2009/apr/09/shipping-p...

The claim is that just 15 of the worlds biggest ships pollute more than all cars in the world combined.

Yup. This is exactly what I was driving at when I said that everyone is ignoring the elephant in the room. You can clean-up automobile emissions all you want. You can even stop flying all the jetliners in the world. Still, by far, the largest source of pollution in transportation are container ships. What they are doing to our planet both above and in water (species pollution) is horrible.