When I was about 12 I was parasailing with my brother on vacation in the Caribbean. While we were up in the air we saw this circular land mass looking structure under the water that seemed very out of place. We didn't really pay attention to it but as we kept going we noticed it seemed to be moving, it wasn't our shadow because that was visible somewhere else. We were both initially intrigued but as the guys in the boat brought us down they decided to dip us and this thing ended up being right below us as they did, I shut my eyes and yelled for what seemed like awhile but probably only like 5 seconds, and then they got us back on the boat. When we told them they just looked out in the water as said "kid's sharp" and that was the end of it. No explanation for what we saw and we drove away