Well it was taken seriously... it just got off track.
This question has been asked 50 or 60 times here. I'm tired of typing, at least, my answers. Hit search and enter leak.
If you're not willing to get the floor out of the way to buck the rivet then then only choice is to drill it out and use a blind "tank" rivet in the hole... or do as duckdog stated smear some 3M5200 over it...or Sikaflex 592.. or Sikaflex 1a. All will need you to scotchbrite the area around the rivet real good and clean with acetone if you expect it to stick for any length of time.