The amount of crud from ships is shocking. I think it's partly cruddy bunkers and partly limited (any?) legislation on ship emissions. You can see a layer of yellow brown haze over Pompey and Soton some days.
The goal in marine fuel oils seems to be to build an engine that will burn the cruddiest, and therefore cheapest, fuel. The heaviest stuff isn't even distilled and is literally the crap left at the bottom after all everything else has been driven off and turned into petrol, road diesel, heating oil etc. As this has undergone no refining, it contains all sorts of nasties like cadmium, sulphur, you name it.