I did this on the INSIDE of an older aluminum boat I had and it worked great. I had a 4" crack on the bottom that was leaking like a sieve. I sanded the aluminum to clear it of paint, put a fiber glass patch on it and it did the job. It help up for 3 years, then I sold the boat. The advice I got at the time was that this would work if it is in a spot that does not flex too much. Let me know how it works out for you if you try it.
If it's rivets and seams that need sealing, "Gluvit" is a flexible epoxy sealer that you paint on the outside. It works FANTASTIC. Procter marine sells it. I refinished a leaky aluminum tinny with it last year, gave it a coat of marine paint and it looks like a new boat (and it doesn't leak).
hey there I sealed my 14 ft I had. i got aproduct called gluvit from proctors marine. I buffed the bottom clean then you mix the 2 part and apply with brush. only cattch is it has to be painted over because it is not uv resistant. the stuff is like a epoxy I liked the way it turned out and didn't add a ton of weight to the boat either.
Structural urethane works very well. This is the stuff that they put windshields in with. I fixed two tinnies this way and it worked on one with a ½ wide gash and an aluminum patch over it. Stuck and remained flexible. No leaks. Make sure the hull is clean and warm and heat the tube of urethane.