Looks like a Starcraft hull ... is it?
They didn't use pressure-treated wood, but didn't seal the wood either, so I'd say that was a boat that has seen saltwater use and waves over the transom saturated the wood with saltwater, which caused the aluminum skin to corrode. The tin was also not primed, like zinc chromate, which can help prevent corrosive elements from setting up.
That corrosion, which becomes a calcified corrosion layer in the precipitate INSIDE the thickness, will not stop until the transom is fully rotted away :shock: .
Take a look at the 16' Starcraft hull in my profile. I ended replacing the complete inside & outside aluminum skins, plus with a new transom core. Lots of riveting ... but thank God I had bought the boat cheap! ... plus have the skills and/or aptitude (aluminum forming) to fix it myself.
The picture to follow shows a liquid salt 'goop' still weeping out of the wood 2-years after the boat had last seen the water (bought the hull from a widow).