The bunker fuel used by cargo ships is a byproduct of the refining process after they extract the higher quality fuels.

The fuel has to go somewhere. We could bury it back in the ground because we don't want to burn it, but we need some fuel to power international trade.

In terms of energy per ton mile, you can't really beat a cargo ship.

Horribly long link from Google, pdf warning.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://...