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]