I had a similar problem. Went to change under water pool light, so turned off light switch, and breaker to GFCI circuit. Then measured no voltage black to white and either to ground. Safe to proceed right? There I was no shirt, tits on the wet ground removing pool light bezel. As soon as I detached it I started sizzling. Turned out the power coming off the utility pole had the bare neutral wire severed by a tree branch. The house ground was my pool, and the connection was at the light!! Any resistance in the neutral wiring back to the utility transformer can put voltage on neutral. Since neutral and ground get bonded together at main box, having a volt or two difference between pool ground an power ground is possible especially if the pool is far from the house. Sometimes the difference can be traced to a well pump that has lost its insulation, and is supplying current into the soil surrounding the well. Just some random thoughts! Jim