The Collaborative International Dictionary of English v.0.48:

Seashore \Sea"shore`\, n.
   1. The coast of the sea; the land that lies adjacent to the
      sea or ocean.
      [1913 Webster]

   2. (Law) All the ground between the ordinary high-water and
      low-water marks.
      [1913 Webster]

WordNet (r) 3.0 (2006):

seashore
    n 1: the shore of a sea or ocean [syn: seashore, coast,
         seacoast, sea-coast]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "seashore":
   aquatic, balneal, bank, beach, berm, coast, coastland, coastline,
   deep-sea, embankment, estuarine, foreshore, grallatorial,
   ironbound coast, lido, littoral, natant, natatorial, natatory,
   plage, playa, riverside, riviera, rockbound coast, sands,
   sea margin, seabank, seabeach, seaboard, seacliff, seacoast,
   seaside, shingle, shore, shoreline, strand, submerged coast,
   swimming, tidal, tidewater, water-dwelling, water-growing,
   water-living, water-loving, waterfront, waterside