OK... It is merchant ship, not cargo ship, (unless referring to a ship of state) The people who work on merchant ships are called merchant mariners or merchant seafarers (merchant seaman is gendered) The merchant marine is the industry merchant mariners work in. There is no s on merchant marine

The term merchant marine refers to private industry. It is not a government agency or branch of the military. Flag state of a merchant ship refers to the country the ship is registered in, not the nationality of the owners.