WordNet (r) 3.0 (2006):

seawall
    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:

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