In BE the term Merchant Navy is common (Merchant Marine is not), and of course the word Merchant is necessary to distinguish it from the military navy, generally referred to either as simply the Navy or, more properly, as the Royal Navy.
The term is familiar to me, but I wondered if it is still in use.
I checked, and it is:
The Merchant Navy is the name given to the UK's commercial shipping industry. It is made up of shipping companies that own and manage a wide range of different types and sizes of ships. http://www.careersatsea.org/