sea rover

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. One that travels extensively by sea.
• n. A pirate.
• n. A pirate ship.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. A herring.
• n. One who roams about the ocean much of the time.
• n. A pirate, buccaneer or privateer; an ocean-going marauder.

from the GNU version of the Collaborative International Dictionary of English

• One that cruises or roves the sea for plunder; a sea robber; a pirate; also, a piratical vessel.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. someone who robs at sea or plunders the land from the sea without having a commission from any sovereign nation