Finney, the holes for the sending unit screws are molded in, not drilled, and the bottoming barrel nuts are molded in as well. Not the same as a drilled hole. To accomplish the same effect, you'd have to weld the plastic, which means pulling and evacuating the tank, then filling with an inert gas.
Hermco's link is good stuff. I that stuff used it to patch a hole in the gray-water tank on my camper after a piece of road debris punched a 3/4" hole in it. That was over a year ago. No sign of it letting go or loosening.
For smaller holes, there is a patching material made for polyethelene Kayaks that is amazing stuff. You clean the surface with alcohol or xylene, and then brush this stuff on, then heat it with a hair dryer. It is chemical resistant, and I bought this to have so that I could repair my wife's Kayak if I needed to, then used it to repair a crack in a 5 gal. diesel fuel can just a week ago.
Polyethelene Patching material
After all that, I'd be looking for a replacement tank and consider all repairs temporary.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury