A merchant vessel is a ship that transports cargo or passengers. The closely related term commercial vessel is defined by the United States Coast Guard as any vessel (i.e. boat or ship) engaged in commercial trade or that carries passengers for hire. This would exclude pleasure craft that do not carry passengers for hire or warships.
They come in a myriad of sizes and shapes from twenty-foot inflatable dive boats in Hawaii, to 5,000 passenger casino vessels on the Mississippi River, to tugboats plying New York Harbor, to 1,000 foot oil tankers and container ships at major ports, to a passenger carrying submarine in the U.S. Virgin Islands.
Most countries of the world operate fleets of merchant ships. However, due to the high costs of operations, today these fleets are in many cases sailing under the flags of nations that specialize in providing manpower and services at favourable terms. Such flags are known as "flags of convenience". Currently, Liberia and Panama are particularly favoured. Ownership of the vessels can be by any country, however.
The Greek-owned fleet is the largest in the world. Today, the Greek fleet accounts for some 16 per cent of the world’s tonnage; this makes it currently the largest single international merchant fleet in the world, albeit not the largest in history.
In English, "Merchant Navy" without further clarification is used to refer to the British Merchant Navy; the United States merchant fleet is known as the United States Merchant Marine.