The Collaborative International Dictionary of English v.0.48:

Seafarer \Sea"far`er\, n. [Sea + fare.]
   One who follows the sea as a business; a mariner; a sailor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seafarer
    n 1: a man who serves as a sailor [syn: mariner, seaman,
         tar, Jack-tar, Jack, old salt, seafarer, gob,
         sea dog]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "seafarer":
   AB, Ancient Mariner, Argonaut, Dylan, Flying Dutchman, Neptune, OD,
   Poseidon, Varuna, able seaman, able-bodied seaman, bluejacket,
   buccaneer, deep-sea man, fair-weather sailor, fisherman, hearty,
   jack, jack afloat, jack-tar, jacky, limey, lobsterman, mariner,
   matelot, navigator, pirate, privateer, sailor, salt, sea dog,
   sea rover, seafaring man, seaman, shipman, tar, viking, water dog,
   whaler, windjammer, windsailor