Let me tell you my story I had a heater doing the same thing and in the process of diagnosis just like you I kept putting it in the water and watching the GFI Outlet keep popping until the one time I put it in there and it literally blew the heater apart shattering the tank. Be carefulI guess I was just curious as to how it was tripping a circuit it wasn't on. Must be leaking voltage into the water and getting picked up somewhere thus tripping the circuit... one thing after another...