Oh btw, lots of people asking for suitable location for a main base, i bet what you actually want is 'Safe bases' avoiding night assault raids, not bothering with base defense combat and repairing things.
One of the basic things you can do is finding a safe point at some place at the beach then putting some ground tiles just to walk on, creating a narrow corridor towards the sea, right above sea level (outside of the island, above the sea, this way raiders won't be able to spot you, if you go further to the sea long enough there will be an error and raids won't even happen)
One example for a total safe spot: from your default starting place travel to the east alongside the beach, right before the bridge you will see a high ground, right at land and sea meets. (it is the intersection point of river and sea)
Just go top of it and start placing tiles towards the sea, to southern direction. You won't even need columns or something like that, just ground tiles,keep placing them until you see some rocks in the sea, going that much further means total safety for any type of base. It is totally ugly but also functional. Don't forget placing walls, otherwise you or others in your base may fall into sea.