The stickiest stuff for water I have seen is the polyethylene film tape with supersticky glue, the newer sail-repair tape.
I got a big roll of some commercial stuff .010 and .020" (thats thousands of an inch, stupid measurement system) 5 years ago. It is unchanged, is excellent sail tape, and sticks to lots of stuff, does not curl (and does not leave nasty duct-tape-type residue). MAYBE it can be put over a mechanical patch (use 2 or 3 layers?) if so, it will keep water OUT.
But...How do you get good tape onto the cleaned-up inside of a poop tank? Eeeww!
BTW- NEVER put gasoline mixed with ethyl alcohol (gasahol) into a polyester resin/fiberglass tank (or parts), as the polyester plastic will dissolve. This problem is likely to come up again, as gasahol reappears in the USA after a long 20 years.