If you go with a floating dock, a simple idea would be to use a fence post driver to set two 1 1/2" or 2" pipes (common iron water pipe) into the river to serve as the end supports for your dock. On shore you would want to bury 2 cement blocks with a 3/16 or bigger stainless steel cable attached to each.

: to be continued in part three.-if that gets through the spam filter