Well what happened was that I left it out for several seasons by the water. Somehow I guess through freezing and thawing water found its way in. There are two small drain plugs on the hull, and maybe water found its way through them. It got so waterlogged I couldn't even push it into the water (It was parked on the edge of a beach). It took myself and two other people to tip it upright to drain it. However, it wouldn't fully drain. I had to cut the drain holes wider and was able to drain it out, but the damage was done. I got it out of the water and onto a trailer and have kept it garaged for several years now, but it still ways a ton. It works fine, and I used it for several years since recovering it. It just doesn't suit my needs if you need to trailer it everywhere.
Not knowing the boat but I do have some experience with waterlog boats and plastic boat repair. This one sounds like it is constructed with foam in the hull. Once that foam gets soaked, it stays soaked and VERY heavy.
For whoever grabs this up my guess is that you'll want to cut several 4-6" holes strategically in the hull and use your hand and a scraper to hog the waterlogged foam out. What you can't reach will dry to some degree if you leave the hull open in a warm dry area for a month or so. Once you've got it lightened up, You can add new foam in the way of Packing foam peanuts (AKA: "ghost turds") sealed in trash bags. Finally, you'll need to patch those holes. Beckson deck hatch's are nice as they allow you to get back in AND allow you some storage. However if the area isn't flat, you'll have to glue the outer hull pieces back in or install deck hatch's. ( Jamestownmarine.com in warren RI has everything you need but be warned the G=Flex plastic boat kit needed is $40 . Worth every dime though!
https://www.youtube.com/watch?v=2a5RlcP-4JE