Plain old two part epoxy. Mix the two part along with a couple of "glugs" of Acetone. It will become a runny mixture that when poured onto the wood, will soak in. The acetone will evaporate, then the epoxy will harden. Can be sanded and painted, or whatever you wish to do with the area.
I used this method when I replaced the flooring in our Caravel
. The plywood was checked just inside the door. Repair has held for over ten years.
This will not work with rot, only with plywood which has been exposed to water and has become checked and separated which occurs near doors or other places where water tends to get into the plywood sub floor.
This method can also be used to repair plywood which has become d-laminated. In this case, use a 5/16 or 3/8 drill to make holes in the area. Put a stop on the drill bit so as not to drill through the bottom or last layer of plywood. Mix the epoxy and acetone mixture and pour into the holes, saturating the area. Next morning the plywood is better than new.
A couple of years ago I had to replace the very rear portion of the sub floor in the Caravel
. I used this mixture to treat the new peace of wood to prevent such damage in the future to the new piece.