The salt from sweat builds up in the hot tub.
The off-the-shelf quasi-portable hot tub violently pumps this ionized fluid all too close to the motor pump fields.
Any time you have charges forced through a magnetic field you'll get energy transfer.
This particular effect is known as capacitive coupling though if I were to name it I'd call it something like the Hall effect.
If you get the installation instructions for the hot tub they are very specific that the GFCI must be a fully grounded one, not merely a fix for a two wire circuit.
For the particular hot tub I worked with, I had to run a separate grounding conductor back and bond it to their continuously exposed copper main water supply which in turn was bonded to the GEC. Immediately after that the water voltage went to zero.
Greg is witnessing the same phenomenon with his 'stray' voltage. It must be bled off. Further efforts to isolate and insulate will merely permit the voltage to jack up to astounding thresholds.
What you are witnessing is a secondarily derived power supply right into the bath of slightly salty water.
This aspect of hot tubs is not well explained in the customer literature but should be. At some point someone is going to get hurt and lawsuits will flow.
