I fixed my TheMightyHobie18 race boat with this exact problem: I had fallen heavily on it during a tack, and the soft spot grew across the whole panel. I set my SkilSaw to 1/8 inch of blade showing, & sawed a large straight-edged rectangular hole just thru the skin. Stay inside of the stock patterned surface: like about 1 1/2 inches from gunwhale (curved edges) because there is no foam sandwich underneath there.
Carefully peel off the skin from the foam because you will glue it back down later- use handsaw, bare hacksaw blade with tape on it, a long chisel or??
If the foam has turned to brown crumbly sawdust, then the outer skin and the underneath layer are not stuck to each other anymore: the rest of the boat may have the same problem.
Mine showed white foam but it was torn: I had to cut and sand it out, a big job to get neatly done. It was replaced with thin 1/8 thick surfboard foam sheet glued in with epoxy thickened with microballoons. When flat and curved to fit the deck curve (a BIG job sanding/filing with Shurform tool), then the skin was fit back into place. The skin was glued down as above with polyethylene sheet, thin plywood and cement blocks to press it down in place.
When hard, fill your saw kerfs with epoxy thickened with fumed silica gel. It will be hard for many years.
Then you can think about doing the other decks, caused by the boat sitting uncovered in the sun, full of water.
