I had a similar problem with an aluminum 16 foot flatbottom. My buddy and I pounded each and every rivet together on the entire boat. 1 person take a hammer and pound the head side of the rivet as the other person presses something firmly against the other ended of it. This will draw the rivet back together. My flatbottom was leaking like crazy and after we spent hours doing this, it was incredible how much it helped. It wasn't perfect, but it was 90% better.
After we put it back in the water, we realized it was still leaking. Put it back on the trailer, filled it with water and got underneath to see what rivets were still dripping. We tried to pound them together again. Some would take, some wouldn't. The ones that didn't we put some liquid steel over them. The thing is dry as a bone today.