This is a sloppy method, but if you try it, it should fix the leaks.

First put enough water into the boat that the level is above the normal water level on the outside. Watch where the leaks develop and mark each rivet. If the rivet is really loose replace with another rivet or small bolt. When you replace rivets or want to seal a somewhat leaky one, try JB Weld. The stuff is epoxy based and will do the job.

The primary consideration is to find the source of the leak and deal with it instead of the entire bottom. Gluvit or other epoxy cures will do the job, but I don't think they are as efficient as finding the source and curing it there first.

Keith