Originally posted by Cap'n Bells:

My experience ..waves seem to go to a height irrespective of land (ie the wave will only got fot example +25 from sealevel if u dig down to +24 from sealevel you will sometimes get waves inland. so sadly building earth walls doesnt make it a seawall :-(

This. The water has a level, and it doesn't matter where you are north/south or east/west, sea level is sea level. "Inland" just means "not close to the edge of the island". Regardless of how far you are from the "sea", you can dig down with the pick and find water, if the area is low enough.

Storms make the water go up and down, also known as "waves". The only way to keep those out of your house is to build higher.