I had a "tin foil" boat years ago. 14 footer with a thin aluminum skin I could pick up and hold over my head. Probably sold as a cartopper. I got it for nothing, looked like it had blown off a car top. Back corner all rumpled, rivets loose or popped out leaked like the proverbial sieve. Took it to a local pond to mark leaky spots after I had fixed the obvious spots and had to pull it before I got 'em all marked. It was that bad.
Solution was two parts: First, missing or severely damaged rivets were replaced with aluminum machine screws liberally schlorbed with JB weld. Second, any leaky seams or other areas were sealed with a product I got at Fleet Farm called Geocel. It is an "elastomeric polymer" similar to silicone but much stickier, cures somewhat "harder" yet still flexible (this is important in a metal boat) and really does the job. This stuff is so sticky... How sticky is it? So sticky it can applied under standing water. No BS.
Upshot is I got that boat totally dry, used it for 4 years and sold it for $100. It was an interesting and telling project that sold me on the Geocel stuff. Look for it with the rest of the sealers and caulks in a cardboard tube. Bought a similar product more recently called Lexcel, same kind of sealer only I found it in a smaller conventional tube like a single serve tube of silicone for your tub.
Take heart, it can be done!
