Some pictures would be good. They are pretty strong boats so replace like for like. If it’s 10mm ply use 10mm, 18mm then 18mm. An exploritary hole might help. Cut it the size of a hatch just in case you were mistaken.
I don’t know the construction of your boat but a wet deck might have transferred to the stringers and it’s still common practice to butt the untreated stringers into the transom and glass over them all. So you might be faced with a whole tear down.
If you find the deck is the isolated issue.p and it’s a small area, Cut it out until sound, replace like for like with ply glassed both sides, bond it down on binding paste or wet csm possibly, glass all over and job done. Ok it is a scary job really.
Cutting is the only way to find out. I used to grind damage out, now I try to use a small circular saw, with a significant taper to the cut. Set depth, be mindful of where the hull is and cut it out. The advantage of an angled cut is when you cut a replacement part the same size with same taper. It should fit back in the hole without falling through, handy if you don’t have access to the back side