ChrisJ800
I live in a coastal area and see neglected mooring buoys sink then float. The weight of barnacles and mussels eventually causes the buoy to sink. Then after a few months some of the barnacles and mussels drop off and the buoy surfaces again. Then the cycle starts again. Maybe better nutrients near the surface causes the barnacles and mussels to grow again and then it sinks again.
There is a significant difference between your mooring buoys and a floating flaperon. Your mooring buoys have a chain attached to them, which when sinking, more chain will lie on the seafloor and significantly reduce the floating weight. Therefore a sinking buoy will reach a point of submerged equilibrium.