"Merchant shipping" refers to the transportation of goods and products by ships or vessels owned by private companies or individuals. It involves the commercial movement of cargo across oceans or waterways for purposes of trade. Full definition
As such, the exact constraints depends on either the flag state or whatever any nation feels like enforcing on merchant ships in international waters. (politics.stackexchange.com)
Some have the colors of merchant ships; others are flying the king's standard. (bitcoinist.com)
Footnote 85 to a law review Note (i.e. student written law review article) by Maria Efaplomatidis from 2000 lists a number of international treaties that govern pollution from merchant ships. (law.stackexchange.com)