HI Donk. Jas is right. Rebuck the bad, old rivets. This really just entails having a partner hold a hammer on one side of the rivet while you hit it from the other side to tighten it back up (i.e. rebuck it). You can also drill out any old/bad rivets and replace them all together. Blind end pop rivets work well and are readily available. Gluv-it is a liquid epoxy with the consistency of honey that you paint along the interior, riveted seams found below the waterline of the boat after they've been thoroughly cleaned. It will get in to every nook and cranny of the seams yet dry flexible an strong. It does need some UV protection so you'll either need to paint it, or put a floor over the top, which is what I think you're doing. Be sure you never use pressure treated lumber and do use stainless steel hardware throughout (excluding any aluminum replacement rivets, of course). Here's avid that can show you how to do the rivets. Any small, non-rivet holes can usually be patched up by a welder for a few bucks, or you can use products like JB weld stick, marine epoxy for hole smaller than say a quarter or dime.
http://www.youtube.com/watch?v=L0MA1-5NXjs
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