Low water, close to existing marker, 10-15 mph and my guess is broken off marker bouy. Extremely difficult to locate as you are looking for the top of a 4x4 probalby 4 feet below the surface. The slim bouy and narrow lower unit in perfect alignment and bang.
Yes, I have done it before in another location a couple of years ago. Bizer was able to locate the item and notified MP and I think it was removed.