breakwater

Short definition from Wiktionary

Noun

1. 1
   A construction in or around a harbour designed to break the force of the sea and to provide shelter for vessels lying inside.
2. 2
   A low bulkhead across the forecastle deck of a ship which diverts water breaking over the bows into the scuppers.
3. 3
   A wooden or concrete barrier, usually perpendicular to the shore, intended to prevent the movement of sand along a coast.