The main difference in any country is mission. The "Navy" is a
branch of the government in questioned who uses military built
ships as weapons. The Merchant Marines or Merchant deal in Sealift
or moving materials from one place to another. The Merchant Marine
may also defend the Merchant ship he or she is on but the ship is
not necessarily used by the military. Now in the US Navy we have
MSC or Military Sealift Command. MSC moves materials around the
world for the military but is more of a merchant Navy. Also the
merchant navy may have military sailors stationed on them such as
many of the US Navy's oilers.