I did this project
I had a saggy deck on my C22. I used Rot Doctor epoxy on it. In my case the core was not rotten but the upper glass had delamed which created the flex. I did the repair from below since fixing the deck, especially non-skid, is very difficult to have look good. I drilled about 6 holes around the weak area (1/2" holes) with the holes up through the core but NOT through the upper glass. Then I used an oil/grease gun to pump thin epoxy into the void. I used a rubber cork with a hole drilled in it through which I passed the gun's nozzle. I cut the nozzle tip at an angle so the epoxy came out the side of the nozzle while the tip pushed against the upper glass. Pumped a bunch of epoxy up there, corked all the holes, and then put sand bags on the deck to compress it all together. Solid as a rock.If you go this route I can provide more details. If your core is completely rotted then you are looking at removing and replacing the core. Still, do the work from the inside and your deck will look good after the project.