The Collaborative International Dictionary of English v.0.48:

merchant marine \mer"chant ma*rine"\, n.
   The ships owned by nationals of a particular country that are
   engaged in civilian commerce; also, the personnel operating
   those vessels. Distinguished from the navy, which contains
   the vessels of war.
   [PJC]

WordNet (r) 3.0 (2006):

merchant marine
    n 1: the crew of a merchant vessel
    2: conveyance provided by the ships belonging to one country or
       industry [syn: shipping, cargo ships, merchant marine,
       merchant vessels]