The earth is the only planet in the Solar System on which water exists in three forms - ice, liquid and vapor. The reason it is anything other than ice is because of sunshine. The Sun not only warms the land but evaporates water from the sea and powers weather systems so that it rains back down on the hills. The winds that whip up waves are also indirectly caused by solar power. The force of the water in a waterfall or a crashing wave represents a tremendous power.
Worldwide, hydroelectric power accounts for as much energy production as nuclear power, and could provide a lot more. Every meter of the North Atlantic coastline of Europe receives an average of 50kW of power in the form of waves. Water can quite literally, in geologically short timescales, move cliffs and mountains, wearing them down, grinding them up and washing their remains away.
The line where land meets sea stretches for hundreds of thousands of kilometers around the world. Water may appear to be a soft chisel but it never fails to find the weakest points in rocks, splitting off boulders and cutting caves and arches through the headlands.
Waves break onto a shore in a circular motion, throwing sand or stones up the beach then dragging them back. If there is a current along the coast, the sand or stones zigzag their way with the current gradually stripping the beaches and building a long spit downstream. Over geological timescales sea level has varied by hundreds of meters, leaving raised beaches half way up present day cliffs, drowning valleys once occupied by glaciers and turning river valleys into natural harbors. In some places cliffs are being washed away into the sea faster than humans can defend them.