I had one of those unstable stone blocks in my home as well. It is caused by support problems, but it won't show that (nothing is red, and in my case, I tacked 5 more stone blocks on top of these). It may be some sort of round-off or rare math bug. I fixed it by just putting 2 blocks buried into their neighbors where one used to be.

as for wave damage, its a live and learn thing. Next time you build on the coast, you now know to wait for a storm to get a feel for how high you need to raise the ground. Or just raise it about 3 vikings tall.