Ships use heavy fuel oil, which is the dirtiest transportation fuel in use today.

https://en.wikipedia.org/wiki/Fuel_oil#Bunker_fuel

Cheap, yea. But also extremely dirty. What is the mpg ona tanker using crude diesel again? Cool, now swing that tanker three or four times through Singapore.

Okay, but you have to divide by the entire cargo of the ship, which is quite enormous. So I still don't think it's something easy to estimate without doing the math.

Cargo ships are flagged in countries of convenience and don’t meet many standards.

The top 15 biggest polluting ships produce more pollutants like sulfur oxide than all cars

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://...

It's way better than anything else. Seriously.