Put one of these on each corner that is closest to the shore.
Use the corresponding inner corner to sandwich the board between.
Put one of these on the 4x6 to anchor the cable to.
Use the corresponding flat plate to sandwich the 4x6 between.
You need to use the plates and inner corner to prevent sucking the carriage bolts through the wood, and possibly splitting the wood.
Run a 3/8" dia cable to a 4"x6" that is sunk in the ground and concreted in. Use a long turnbuckle to take up the slack.
The parts are found at Dockbuilders:
https://www.dockbuilders.com/floating-dock-hardware2.htm I'd run the cables at a 45° to the posts.
