This kind of erosion is called 'coastal erosion', usually caused by waves or high winds. Abrasives in the water or impact against the rock can cause it to chip off or degrade over time. One of the causes of coastal erosion, which might best fit your question, is hydraulic action. This describes the impact of the wave hitting the surface of the rock or cliff. The wave compresses air inside cracks on the rock, and the pressure causes the rock to splinter off.