According to the EPA,

On October 9, 2008, the Member States of the International Maritime Organization adopted new international standards for marine diesel engines and their fuels.  This new program will dramatically reduce air pollution from ships by establishing a new tier of performance-based standards for marine diesel engines and their fuels and by establishing additional, more stringent emission requirements for ships that operate in specially-designated coastal areas where air-quality problems are acute.

EPA announcement and resources here. 

Per International Custom House Blog Here:

“Implementation of these IMO standards will be a very important part of a coordinated national strategy to reduce emissions from ocean-going vessels,” said Rear Adm. Sally Brice-O’Hara, the Coast Guard’s deputy commandant for operations. “I was fortunate to personally be at IMO when these important amendments were adopted. I also had the distinct pleasure to deposit the instrument of ratification for MARPOL Annex VI, on behalf of the United States.”