The United States Merchant Marine is an organization composed of United States civilian mariners and U.S. civilian and federally owned merchant vessels. Both the civilian mariners and the merchant vessels are managed by a combination of the government and private sectors, and engage in commerce or transportation of goods and services in and out of the navigable waters of the United States. The Merchant Marine primarily transports domestic and international cargo and passengers during peacetime, and operates and maintains deep-sea merchant ships, tugboats, towboats, ferries, dredges, excursion vessels, charter boats and other waterborne craft on the oceans, the Great Lakes, rivers, canals, harbors, and other waterways. In times of war, the Merchant Marine can be an auxiliary to the United States Navy, and can be called upon to deliver personnel and materiel for the military.
In the 19th and 20th centuries, various laws fundamentally changed the course of American merchant shipping. These laws put an end to common practices such as flogging and shanghaiing, and increased shipboard safety and living standards. The United States Merchant Marine is also governed by more than 25 (as of February 17, 2017) international conventions to promote safety and prevent pollution.