The ocean is a violent and place. As you lay on the beach enjoying the warm sun and the salty air, so much brute force creates the warm sand in between your toes. Centuries ago, there were stories of sea monsters that could quickly destroy an entire ship. Modern day research showed that a naturally occurring phenomenon called rogue waves may have been mistaken for a sea monster.
Since these waves were so destructive, people built cities in bays and behind offshore islands. But even with a barrier of land, the ocean can still grind away at everything in its path. It takes more time to turn a pier into drift wood, but it will happen.