I was doing some winter maintenance on my boat and unhook the cranking battery then later when I placed the positive cable on the battery post there was a arc, so something is pulling power even though the key switch is OFF. Is this normal to have the positive side arc when the key switch is off? I was thinking about installing a master battery on/off switch the kill all power to the circuit but if it is important enough to have then why wouldn't the boat manufacture install one at the factory, this is a fully loaded 2018 Pro Guide boat?

I think the radio or the GPS puck are the culprits pulling constant power, would these pull enough to drain a starting battery after months of winter storage?
Should the GPS puck have constant power?

Any recommendations on battery on/off switch? Thinking about one for the starting battery, one for the electronics battery and one for the two trolling motor batteries, or is the latter two a waste of money?