Most of the soot is a byproduct of the "heavy bunker C" fuel cruise ships burn when legally allowed. This is not diesel. Ships have to use diesel in certain territorial waters and in ports. But when out in international waters they switch to the heavy bunker C fuel, which is also called #6 fuel oil. It is cheaper. But it is the dregs from a barrel of crude oil. It has to be heated in order be fluidic so it can get to the engines and flow through the injectors. You can see how this causes build-up in the exhaust stacks as well as the carry-over your buddy refers to.