Huge amounts of cargo, from oil to t-shirts, are moved across the oceans by ship. So far, there has been little effort to deal with the emissions from these ships that generally burn some form of petroleum-based fuel. There are many nuclear warships, but private corporations have not invested in nuclear-powered cargo ships...yet.
Here's a brief look at commercial nuclear vessels. Now the EPA and Canadian government are beginning to move on controlling the emissions of ships within nationally-governed waters.