I'm in the United States. I have a handful of three-prong outlets that have been grounded to the underground water main just as it enters the crawlspace. All of the ground wires are home runs to the same clamp on the water main. This rules out ground-to-ground potential differences but maybe not ground-to-neutral differences, since I believe the panel is coupled to the water main only through dirt.
It's my understanding (correct me if I'm wrong) that the ground terminal on GFCI outlets can safely be left unconnected as long as the cover plate is marked "NO EQUIPMENT GROUND." Is this safer than the existing plumbing-ground situation I described?
I realize that neither choice is ideal, but replacing all of the 14-2 NM with modern 14-2/WG isn't an option at the moment.