Ships use a less refined fraction of the oil barrel. There are no regulations for air polllution at sea, they dump their trash out beyond 10 miles.
Diesel was cheaper than gasoline for land based vehicles before the EPA regs took out S. Since then it has mostly been more expensive or similar to gas. Economies of scale may reduce the cost of refining diesel. World geopolitics probably will have more to do with fuel prices than taking out S.
The current trade wars and tariffs started by Trump are bad for international supply chains of materials. It may negatively impact the cost of diesel more than taking out the sulfur.
