I’d use the polyurethane rather than traditional varnish. Any solvent-based poly should work. I treated a transom board for a small aluminum boat with the old timers formula (poly-mineral spirits-boiled linseed) and it really does soak into the wood much better than a surface application of...
We boat on the Mississippi and Illinois rivers mostly, and we've had good luck with our Humminbird Helix 5 with CHIRP & GPS. I bought a separate SD card that has detailed maps of Midwest rivers and lakes. It's definitely worth it. It includes all sorts of details like underwater cables, sunken...
I have to agree with those who say the transom (and probably stringers and other structural elements) are rotten. To make the boat safe, you'd likely be looking at a gut-and-rebuild.
If you don't want to restore the boat, and if you're getting a great deal and can sell the trailer and part out...
My 19' bowrider came with a single battery, which I did not like. I added a house battery via the Blue Sea Add-A-Battery kit. Pretty easy installation. There are many online vendors that will make custom marine battery cables in just the length and size you need.
I really like the redundancy...
