The first jon boat I had I broke a large section of weld in the bow where the front of the jon boat starts to make the upward curve. The crack was about 6 inches long in total. I cut a patch out of a piece of sheet aluminum and formed it into the proper shape. I the drilled holes on both side of the patch / through the hull and then pop riveted it in place and sealed it up with silicone caulk. It's not as pretty as weld but it works. It made a strong patch plus it never leaked a drop and I owned that boat for about 6 years after that.
As for the above mentioned brazing rods. I would avoid those things like the plague. If you want it welded take it to a shop and have it done right. Unless you already have experience welding aluminum learning on the bottom of your boat is not the way to go. An all aluminum boat dissipates heat rapidly making it near impossible to get the boat hot enough to make the rods work. Leave the torch on one spot to long and you've just burnt a nice hole in your boat. Been there done that, won't do it again.