The Collaborative International Dictionary of English v.0.48:

marine mine \ma*rine" mine`\, n. (Mil.)
   A military explosive device designed to be placed on or under
   the surface of a body of water, and to explode when ships
   pass nearby or come in contact with it. Its function is to
   destroy enemy ships or deny hostile naval forces access to
   certain areas of the sea, usually near the shoreline. Also
   called underwater mine and floating mine, and previously
   referred to as a torpedo (See torpedo[2]
   (a) ).
       [PJC]

WordNet (r) 3.0 (2006):

floating mine
    n 1: an explosive mine designed to destroy ships that bump into
         it [syn: floating mine, marine mine]