My money is on Illudium Q-36 explosive space modulator.
http://www.buoyandbeacon.com/marinebuoysonsale.asp
It says that large buoys will have a steel frame and a concrete ballast.
So this object may the guts of the buoy, where the cover has been broken off, eg smashing on the nearby points rocks.
Further it says the buoy is filled with flotation foam. So that may account for the frame travelling from rocks to beach. The foam then tears off after a bit, and the frame is left behind.
Have a look at how they are made… The buoy has a hole at the top for supporting the flag, light pole, or other structure up above, and to allow inspection and servicing of any stuff inside the buoy’s waterproof shell.
So legs in concrete, frame to hold the buoy in shape, foam around the frame. the top of the frame has a steel reinforced hole for the user supplied or ordered parts… basically one buoy design for many functions.
It looks like the base for a Strange Grid Ball
Based on the images, it looks so me like…
Oh, wait…
That’s what I was going to say it looks to me like. Nice hypothesis in the rest of your post.
Perhaps they had a convenient boulder down under where they wanted to anchor a buoy, so they rigged up an articulated armature that grasps the boulder the way a hand would grasp a rock.