The Collaborative International Dictionary of English v.0.48:

Breakwater \Break"wa`ter\, n.
   Any structure or contrivance, as a mole, or a wall at the
   mouth of a harbor, to break the force of waves, and afford
   protection from their violence.
   [1913 Webster]

WordNet (r) 3.0 (2006):

breakwater
    n 1: a protective structure of stone or concrete; extends from
         shore into the water to prevent a beach from washing away
         [syn: breakwater, groin, groyne, mole, bulwark,
         seawall, jetty]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "breakwater":
   abutment, anchorage, anchorage ground, arc-boutant, arch dam,
   backstop, bamboo curtain, bank, bar, barrage, barrier, basin, beam,
   bear-trap dam, beaver dam, berth, bill, boom, breastwork,
   brick wall, buffer, bulkhead, bulwark, buttress, buttress pier,
   buttressing, cape, chersonese, cofferdam, coral reef, dam, defense,
   delta, dike, ditch, dock, dockage, dockyard, dry dock, earthwork,
   embankment, fence, flying buttress, foreland, gate, gravity dam,
   groin, hanging buttress, harbor, harborage, haven, head, headland,
   hook, hydraulic-fill dam, iron curtain, jam, jetty, jutty, landing,
   landing place, landing stage, leaping weir, levee, logjam, marina,
   milldam, moat, mole, moorings, mound, mull, naze, ness, parapet,
   peninsula, pier, pier buttress, point, port, portcullis,
   promontory, protected anchorage, quay, rampart, reef,
   retaining wall, road, roadblock, roads, roadstead, rock-fill dam,
   sandspit, seaport, seawall, shipyard, shoulder, shutter dam, slip,
   spit, spur, stone wall, tongue, wall, weir, wharf, wicket dam,
   work