From WordNet (r) 3.0 (2006) [wn]:
breakwater
n 1: a protective structure of stone or concrete; extends from
shore into the water to prevent a beach from washing away
[syn: breakwater, groin, groyne, mole, bulwark,
seawall, jetty]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Breakwater \Break"wa`ter\, n.
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.
[1913 Webster]