Weird Thing in the Ocean

On the approx. 90 minute plane ride from Narita Airport (close to Tokyo) to Kumamoto City (closest airport to Goshoura, about 3 and a half hours by car and boat), we saw this huge, weird structure in the middle of the ocean. So far I've asked the flight attendants, my co-workers, and my host family, and nobody knows what it is. (Yes, it does deserve its own page).

Use the boat as reference to see just how huge it is. Even if it's a small fishing boat, the thing is pretty huge, but I'm guessing it's about a 20-30 foot boat.

Close up

If you know what this is, please let me know.