This is how I just plugged some holes.
#1: CLEAN the area the best you can to remove all corrosion that may be present and prime.
Setting solid rivets by hand is labor intensive if you have many to do.
On this project I had over a hundred 1/4" rivets to set. Using a 10# sledge head
and 2# hammer, it takes an average of 4-5 whacks to get it flat. (7-8 if you are finicky about the looks).
If you drill a large dimple in the sledge head to match the rivet head,
it will prevent the head of the rivet from being deformed during the beating process.
But, in my case, it was not an issue, just plug the hole and go fishing !!!.
If you have never done this before, I STRONGLY suggest you practice on some scrap
material before you go messing with your boat.
Any heavy steel item will work for your anvil. I also have a 1"x1"x12" steel bar to get into tight places.
Use your noggin, look around and see what will work for your particular application.
Jus my Dos Centavos