in my case I had a tank fitting that was cracked long ago and weeping slowly. Also a macerator outlet. Cut it off flush with the tank outer wall, then used a hole saw to remove the uneven surface where the spin-welded fitting was. Result was a 3" diameter nice round hole. Used a Uniseal and piece of PVC pipe with glued on end to close the hole. According to Uniseal
Uniseals the resulting connection is leakproof to 65 psi, which I calculated was a minimum of 3 times the static pressure my tank would experience. $8.50 for the Uniseal, $12 for the PVC pipe and fitting. Already had the PVC cement. I figure I both saved $1K minimum boat $, and was able to avoid dramatically reducing my holding tank volume.