First check with volt meter for both AC & DC to see which the 3 volts is. This is not normal. I supect that the 3 volt is DC. Maybe a bad ground on an appliance that is dual voltage, like referigerator?

Roger

That clicking sound is the isolator letting you know there is juice in the ground like you measured.
Pull the shore power and repeat measurements.
If still voltage separate 1 batt at a time while measuring.

I have yet to find DC dropping onto the AC ground. Most of the time the AC is fouled somewhere. Either the plug at either end. I've seen zorched pedestal outlets, zorched shore cord plugs and zorched wiring behind the panel at the helm.
Also had a boat 2 docks down from me with a zorched power cord plug. If the cord was jostled it would randomly put voltage to ground and make my isolator buzz.

If no probs can be found on your boat start pulling shore power on your neighbors. Have someone watch the meter on your boat so you can tell when the problem quits.

Can also measure neutral to ground at the dock pedestal with your boat completely out of the circuit. If your still getting voltage you know the problem is on the dock or another boat.

I always shoot either silicone or contact cleaner/dielectric spray on everything at the beginning of the year and inspect & respray a few times throughout summer.

-Karl

Thanks I will be diving into this electrical problem shortly. Just sold my Sea Ray today and am ready to put my attention into the .