+1 on the caulk. -2 on the welded solution. Aluminum is dirty stuff and prone to leak to begin with after welding. We make aluminum diesel fuel tanks for the military. We even have to leak test the areas of the tank where the baffles are welded. And we find leaks with no visible burn through. I can only imagine how difficult it would be to try to weld a leak proof seam on an old painted boat hull. And even if you are succesfull chances are it would crack from stress long before the caulk would fail. For what it’s worth.

Try a loaf of the cheapest supermarket bread you can find. Wad a slice of it up sans crust and shape it around your hook. It won’t last long (maybe ten minutes), but if you are anywhere near an area that people feed ducks, you won’t be able to keep your pole in the water long enough for that to matter. Also expect to catch catfish and the occasional bullhead. Use the lightest sinker you can get away with, if any.

Edit: I used to do this way back when I was in sales just out of HS to pass the time between appointments. I have literally caught dozens of carp per hour and some of those were monsters.