Water probably wicked into the wood through the screw holes.

Marine ply is built with waterproof glue and there are not supposed to voids in
the wood, so there's a chance the rot didn't spread far beyond the screw holes.

Even so, that won't matter much. Odds are you currently have screws just
exactly where the screws need to be in order to secure the floor to the
framing. If those points are compromised, it wouldn't matter much whether the
remainder of the floor is good or not. You could fart around sistering the
framing, etc, but that would be way too much work to go through just to try and
save a 20 some year old piece of plywood. Start over. :-)