Every step sounds pretty good in winter - except using a sledgehammer to drive 6x6s into a mucky/gravel bottom.
I also don't think one can sledgehammer 6"x6"s into ground solid enough to support a structure. And putting them into soft muck or mud won't let them hold a dock.
I can use the post driver on the back of my tractor to drive 6" circular posts 4' deep into dry soil. I couldn't sledgehammer them in 2" deep.