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]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "merchant marine":
   Naval Construction Battalion, RN, Royal Navy, Seabees, USN,
   United States Navy, argosy, armada, bottoms, coast guard, division,
   escadrille, fleet, flotilla, line, marine, merchant fleet,
   merchant navy, mosquito fleet, naval forces, naval militia,
   naval reserve, navy, shipping, ships, squadron, task force,
   task group, tonnage, whaling fleet