You need to either tighten or replace the rivets. You can tighten them by placing one hammer (use as a bucking bar) on one side and hitting the other with another hammer or hammer and punch. This will smash them back tight. It's not as difficult as it sounds, but it may take two people depending on the rivets location.
If the heads are sheared off you will have to drill/punch them out and replace them using the above method. When installing new rivets in my boat I put them in "wet" with liquid nails. I guess you could put tar or some other sealer over them to stop the leak but the above method is a more premenant fix.
There is a resin epoxy you can buy from marine products that has to be catalysed.
It is freaking awesome.
Dig out as much old gunk as you can and just lay it over the rivets.
I was amazed with it.
You can build it up pretty darn thick over the rivets.
It stays fairly pliable too.
Some of my rivets were kinda loose too but not anymore. This stuff is crazy good.
It cost me about 40 bucks for a quart.
Trust me it works
Sealant is fine, but Mojo was right on with the need to reseat the rivets.
In fact, buy a riveted boat like a Lund, and if it develops a leak and you send it back to the factory to be fixed, that's exactly what they'll do--reseat the rivets. They might even give you a Lund hat that, if you never wear it, you can trade for a really nice used toothbrush.
Mojo1 is right, reseat the rivets. If the rivets are missing or the heads are missing, drill them out. Clean the area with denatured alcohol. Use a stainless steel screw of the right diameter with washers and a nylon lock-nut and dip the scew in sealant. Your local hardware store has rtv or similar household sealants. A word of caution is that it must be approved for use below the waterline or the water can break it down over time and it will start to leak again. Read the labels carefully! Usually a tub and tile sealant will work, but again...read the label.
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