There have been some posting of pictures and even a sped up video of patching a floor. Without doing the whole thing. Someone may recall the post I can't seem to locate it.
Essentially you make a patch out of wood the same thickness as the floor, trace or better yet cut around edge of patch to get exact match.
Screw a board to the bottom with aluminum foil / plastic wrap / wax paper between the board and the underside of the floor. I can't recall which so FG guys a little help here. This board gives you something to hold the patch up with. The foil etc. keeps the patch from sticking to the board.
Set the patch on the board and fiberglass the patch in with FG cloth and resin. I think one person used Gorilla glue in the seam around the patch.
Once FG hardens unscrew the board you have under the floor holding the patch up. Might put a layer of FG and resin on the bottom of the patch too.
You of course have to remove the seats or cabinets from above the area to get access, and if it is in an area of floor, the flooring will need to be removed.
And if at an edge you will also need to re-apply the fiberglass "tabbing" which is just some fiberglass cloth and resin used like tape to connect the wall to the floor, look under the seats along the edge you will see it. Since it is hidden it's a good place to do fiberglass if like me you kind of stink at it.
This thread was about patching a hole and firming up some soft spots.
Fixing a Floor
And getting the bench seats out.
Scamp- rusty screws and rotten floor!