Best way is to re-set or replace rivets. Sealing them doens't work. They move around too much if they are loose and will eventually just leak again. It is very easy to re-set or replace if you have the floor out. I did my whole boat in the spring. Replaced a ton of them, and re-set any that didn't need replaced. Very easy to do, just need a second person, and the tools to do it.
S.