The Collaborative International Dictionary of English v.0.48:

Seaport \Sea"port`\ (s[=e]"p[=o]rt`), n.
   A port on the seashore, or one accessible for seagoing
   vessels. Also used adjectively; as, a seaport town.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seaport
    n 1: a sheltered port where ships can take on or discharge cargo
         [syn: seaport, haven, harbor, harbour]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "seaport":
   anchorage, anchorage ground, basin, berth, breakwater, bulkhead,
   dock, dockage, dockyard, dry dock, embankment, groin, harbor,
   harborage, haven, jetty, jutty, landing, landing place,
   landing stage, marina, mole, moorings, pier, port,
   protected anchorage, quay, road, roads, roadstead, seawall,
   shipyard, slip, wharf