You need to be out in the school playground for this.

Get an old chair and place it in the middle of the playground. Now fill a bucket with water and throw the water at the chair. You should see how the force of the breaking waves can easily move heavy objects.

Now re-fill the bucket with water and tie a short rope to the handle. Get an assistant to swing the bucket as though it were a playground swing.

Gradually make the swing bigger until the bucket hits the chair. This is the force of the wave when it has not broken.

Now you should be in no doubt that the waves are very powerful and could even weaken hard cliff rocks.