Heavy marine fuel is a great low cost place to concentrate sulphur to balance refining runs where sulphur is being removed from higher fraction auto fuels. It’s cheap becuase they dont have to remove it from the crude and store and sell it, and it means they can access cheaper crudes which tend to be high in sulphur, and so can produce a higher value product.

“you’d want to be a little careful because ships are generally more efficient than anything else, and it would be a shame to see goods moved by road instead.”

In theory you will be taxing per carbon unit – so if ships are more efficient then they will still be taxed less per good.

Just because it may be more efficient doesn’t mean that the best externality tax is zero – it is about setting the private cost equal to the social cost …

You’d want to be a little careful because ships are generally more efficient than anything else, and it would be a shame to see goods moved by road instead.