You're essentially going to have to create a massive "log" of terrain, then tunnel into that log to make a hollow cylinder, then place building pieces on the inside and outside of it to create the effect you're going for if you want it to go under water.
Thanx for the anwser, but it would be cool when water will stop by concrete elements, just like in real life. So Maybye the devs will read this thread and can make it possible. For the rest I Love the final result at this moment.