If you need to replace rivets, do it this way. It is very simple and very effective.
Drill off the heads of the old rivets it they are still there. Do not drill into the hull with that drill bit. You will want to drill the new hole with a different bit.
I used a special pop-rivet on my last boat. I got them at an industrial supply place who carried them. They were an aluminum rivet with a stainless steel mandrel, CLOSED-END. That is, the end of the rivet was sealed. I don't remember the exact size rivet but I believe it was 1/4". I do know that I used a #11 drill bit to make the new holes. This gave a push-fit- I had to push the rivet into the hole- very tight. When the rivet expanded, it locked itself into the hole and became water tight. The only problem was that the stainless steel mandrel was TOUGH and it took a strong grip to pop it. But, once it was in it was very strong. The closed end made for a pretty good looking rivet too- not an open end to catch garbage. I think 100 of them cost me $12 or so. The biggest benefit to using this rivet was that I could do everything from the outside. No need to get back in the boat to buck up the new rivets. No damage to the hull either like dents or anything. The heads were a low-profile wide grip head and actually stuck out less than the factory rivets- very smooth.
I used a locking mandrel Pop-Rivet simaliar to what was mentioned above. Look at Mcmaster-Carr .com. More kinds of rivets thatn you'll ever need. The other thing I did when replacing rivets was to dip the head of the rivet in Permatex to help with the sealing.
If you have access to an Air Compressor, the Air Driven Pop-Rivet guns really pull those things up tight. A lot better than the best hand powered gun. Tin boats sure are a lot easier to repair!