I used to have a 1985 Cat. 25 and it was in good overall shape with no rot in the decks, and it always had some flexing in the cockpit floor, because it is pretty thin plywood core, sandwiched between thin fiberglass. Unless U can verify that there is core damage by sounding the deck & listening for soft spots, I wouldn't worry too much about it. Unless the aft scupper drains are leaking there's no other water penetration source in that area. These smaller Catalinas were made to be lightweight & flexing on the decks is typical. Reinforcement could be done by laminating the plywood with an additional layer beneath the existing cockpit sole, bonded with epoxy, if it is necessary. This would be a job best left to a pro or yard to carry out. Access would be difficult, but not impossible.