Re: 1978 Glastron
If you are looking to just strengthen the deck, then you'll want to cut out what you find that is rotten and replace it with new wood, and then fiberglass over it for strength. I'm surprised that you didn't find a layer of fiberglass under the carpet...perhaps Glastron was just tabbing the edges of the wood to the boat by 78? My 75 had a layer of glass over the whole deck IIRC, with a layer of vinyl over that.
I'm betting that if you wanted to dig further into this boat, you'd find the wood that makes up the stringers to be rotted also.
You have 2 choices. Take the boat down for a season and restore/rebuild the deck, stringers and potentially the transom, or patch in the floor and keep going the way it is.
I'd almost guarantee though, that there is water logged foam logs under the deck ( original flotation foam ) that are weighing the boat down. If you pop the deck, get rid of those things ASAP, they aren't doing you any good full of water.
If you want to see what I mean by taking the boat down for a season, you can check my glastron pages below to see what you might be looking at under that deck.