The latter. They do not help against it. I have a earlier built coastal fortress where the water swaps in the place in front of the building when big waves hit the coast. I do not mind, but it might be immersion breaking if the water happens to float directly into your home.

In my recent coastal base in the plains I raised the ground therefore.

edit: bad english ;-)

lol is fine and yea i just finished part teh wall enough to figure this out man there goes idea of building a base in the ocean