Man it has a lot of burn more so then most salt ones out by me, there may be stray current getting into the water from dockside power. Not sure but it looks like someone caulked around the transom shield it may have been leaking, if it has the wood core may be shot. Take a 1/2 dollar and tap the transom near the drive see if it sounds kind of dull, then tap outward to see if and where the sound changes. You need to do some serious investigation on this one.
Phil
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.