I don't know how far down the rubble goes. But however deep it is, there would be another limestone ledge under it. This marina is in an old quarry on Johnson's Island in Lake Erie. If you are interested in history Google it.
The island was the site of a Union POW camp for Confederate officers during the civil war. In modern use, there was a causeway built to the island and it has been developed for seasonal homes. (Many in the multi-million dollar class.)
Attached is another photo of the quarry side from out on the docks.
Thanks for the ideas. The PVC posts with adaptors sounds like it will work. I first thought of putting brass stakes in concrete but it seemed like a waste of a brass stake.
Starry Night Lighting
"Always look on the bright side of life."