I don't think that the dock will stay put by just ramming it down with a backhoe or whatever, especially in a high water situation.
This might be stupid, but can you weigh it down?
I was thinking the same thing. Instead of hiring somebody to push down with an excavator, just keep bringing more weight out there until it would do the same thing. I don't see how it would stay down long term. Possibly drive another post next to it and tie into it? How about slipping a floating barrel underneath and securing it somehow?
Ps.- Sure glad you didn't get hurt. I don't think I would have even got in the water next to it, let alone try to physically reach in.
