The Collaborative International Dictionary of English v.0.48:

Supercargo \Su`per*car"go\, n. [Super- + cargo: cf. Sp.
   sobrecargo. Cf. Surcharge.]
   An officer or person in a merchant ship, whose duty is to
   manage the sales, and superintend the commercial concerns, of
   the voyage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

supercargo
    n 1: an officer on a merchant ship in charge of the cargo and
         its sale and purchase