Re: how to repair leaking transom
You might want to do the "water test" to see if it really is the transom leaking. While trailered, fill the hull w/ a few inches of water and look underneath at the hull for drips. If none, you'll know the bottom of the hull is sound.
When you find the leak (hull or transom), seal w/ a pop rivet w/ a dab of 3M 5200. Should last a long time.
Previous owner of my boat slathered silicone outside the bottom rivets of the transom brace. It leaked bad when filled w/ water.
Another solution is "Glu-vit" from the inside, if it's the hull that's leaking.
If it's leaking at the transom, also check for wood rot, and treat it before it gets worse. Ethylene glycol (antifreeze) kills the wood rot fungus.