breakwater

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A barrier that protects a harbor or shore from the full impact of waves.

from The Century Dictionary.

• noun Any structure or contrivance, as a mole, mound, wall, or sunken hulk, serving to break the force of waves and protect a harbor or anything exposed to the force of the waves.

from the GNU version of the Collaborative International Dictionary of English.

• noun Any structure or contrivance, as a mole, or a wall at the mouth of a harbor, to break the force of waves, and afford protection from their violence.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a construction in or around a harbour designed to break the force of the sea and to provide shelter for vessels lying inside
• noun nautical a low bulkhead across the forecastle deck of a ship which diverts water breaking over the bows into the scuppers
• noun On beaches: a wooden or concrete barrier, usually perpendicular to the shore, intended to prevent the movement of sand along a coast.

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

• verb show the fins above the water while swimming
• noun a protective structure of stone or concrete; extends from shore into the water to prevent a beach from washing away