Soft Floor

I have an 18' Ken Kraft center console. The floor has started getting soft and after inspecting it I found that it had plywood glued to the underside and it has rotted out. I have my idea to repair it but I would like to hear from some of you guys that have made similar repairs.

Sadly, this is the kind of thing that kills boats. Your rot is not isolated, this is just the part of it that has become obvious. Your choice is to cut out the floor and replace it all, or live with it being rotten and act accordingly. On the bright side, you can start shopping again!
Repair means cutting it away from the sides and ripping up everything. Sometimes it comes apart easy and sometimes it fights you every bit of the way. Crowbars, sawzall, hammers notwithstanding. You may get super lucky and be able to pull the deck skin off the rotten substrate without damaging the skin. That's best case scenario.
Ultimately you will be replacing with new marine ply and then epoxying the deck skin to the new wood. If that doesn't work out then you're in for building up a new layer of glass deck over the wood and then sanding and painting.
As you can imagine, the repair is often more trouble than the boat is worth and you need to ask if you really want to get into that or just scrap it and look for another ride.