We have a 2015 C30. We purchased the boat on Long Island and although it only had 40 hours on it exterior care was apparently not a priority of the former owner.
All that said we initially tied to sand the wood in place. That ended very quickly. We took the plywood down and sanded it in prep for a new finish. You must be very careful not to sand thru the thin exterior wood on the plywood.
I did a lot of research on the best marine coating and chose a 2 part varnish.
It has held up very well over the last 2 summers and looks as good as it did when we put up the refinished wood. I chose to make a template including screw holes of the plywood in case in the future I wanted to replace it with a completely new wood or synthetic material.
One caution, when sanding in between coats of the two part varnish be sure to use brass wool for the final finish prep between each coat.
Good luck.