I can't see the capacitance in the SWA feeder being the culprit so long as everything is securely bonded.
If the feeder insulation was damaged and you had a leak in the drainage system, you might get some leakage into the water. Or maybe if just the outer sheath was damaged and the armor wasn't grounded (I see that a lot) then the capacitance could be enough to give some leakage current into the water. Either way, that could be checked easily by cutting power to the feeder.
Are there any other feeds underground near the drain which could have gone faulty, such as the main to the house? Just a thought.
Just one other point to clarify: When somebody gets a tingle as the water is running away, is this while still sitting in the tub, or while standing on the bathroom floor and touching it?
