Hah, if we're only going to talk only about tiny US ships, run them on whale oil for all I care.
Seems to me the 80/20 here would be to attack the problem near the top of the stack, not the bottom. Those massive heavy fuel oil burning container ships that basically just smog the ocean 24/7 might be a good target for improvements; as well as just general code enforcement.
To be honest, the massive heavy fuel oil container ships are remarkably efficient. The amount of energy they use to move a given weight of cargo a given distance is minuscule compared to that of trucks, and still much better than even trains.
Scale has an advantage all its own when it comes to combustion engines.
For now it seems the improvements (sulphur regulations) only made the situation worse, in term of climate change.