The Collaborative International Dictionary of English v.0.48:

Sea rover \Sea" rov"er\
   One that cruises or roves the sea for plunder; a sea robber;
   a pirate; also, a piratical vessel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sea rover
    n 1: someone who robs at sea or plunders the land from the sea
         without having a commission from any sovereign nation [syn:
         pirate, buccaneer, sea robber, sea rover]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "sea rover":
   AB, Ancient Mariner, Argonaut, Blackbeard, Captain Hook,
   Captain Kidd, Dylan, Flying Dutchman, Henry Morgan, Jean Lafitte,
   Neptune, OD, Poseidon, Varuna, able seaman, able-bodied seaman,
   air pirate, airplane hijacker, bluejacket, buccaneer, corsair,
   deep-sea man, fair-weather sailor, fisherman, freebooter, hearty,
   jack, jack afloat, jack-tar, jacky, limey, lobsterman, mariner,
   matelot, navigator, picaroon, pirate, privateer, rover, sailor,
   salt, sea dog, sea king, sea robber, sea wolf, seafarer,
   seafaring man, seaman, shipman, skyjacker, tar, viking, water dog,
   whaler, windjammer, windsailor