Have you tried to tighten the rivets yet? I only use sealant on rivets that have been pounded flat, and can not be tightened anymore. At that point, it's either replace the rivet, or seal it.
If your only option is to seal, then I would try 3M 4200 or 5200 marine adhesive. You can get it at Canadian tire in the marine section. The stuff that I used takes a good 3 days to cure. But once it's set up, it stays flexable, and doesn't leak a drop.
As for the foam.. That's preference. I would like to have it for safety purposes. But my boat doesn't currently have any foam in it at all.