I sold an old knocker jonboat a couple of years ago, a Fisher Marine that had 'found' too many stumps and cypress knees. Over the years, it had developed some leaks, even a couple of puctures. An old guide from TN told me how to fix it, and while it is not 'permanent' [is anything?] it will last two-three years or longer, and it wonm't leak if you do it right!
Go the the local Home Depot, Lowe's, what ever and go to the gutter section. You'll find a sealent in a tube to fit your caulking gun, this stuff will be a sivery color and it is tough and durable - plus it is and stays flexible! and cheap. Clean all the seams, rivet heads, etc and use a fine wire brush to rough up the metal. Put the sealer down, spread it around a bit and let it dry. I don't recall what type but if you read the label re cleanup you can find a solvent that will help to spread it out and smooth it down. Your now good to go for a year or three, then you can do it again. You know how to do it now! Hope this helps.