Customer is reporting tingling shock in water

Licensed Electrician in SC. Power has been disconnected.

Aluminum dock frame to water: 1.7–1.8V AC with the feeder connected. Drops to zero when the dock cord is fully unplugged (hot, neutral, and ground all open).

Voltage persists with the branch breaker off AND with the service main open, so it's arriving on the neutral/grounding system, not the branch circuit. GFCI upstream never trips, it is fairly new.

Adjacent marina reads higher (2.8–3.1V water to ground pin) with all its breakers off. Lifting the marina's neutral drops it from ~3V to ~1V.

Could this be a utility issue? Would a highly sensitive clamp meter be helpful here? I was looking at the Fluke 368.