Peter, the only safe thing is "eyes and hands", trace out those deck fittings and see where and what they are connected to. It could be that the switch connects your "interface" (charger, invertor, whatever) to 110 or 240 manually, and that all the rest of the ship's systems then run off 110V from that device.
But when you've got a used boat
...you never, ever, know what the POs really did, until you chase down the source of the smoke. I wouldn't trust the PO, or any schematics on the boat, until I'd actually verified it. Shouldn't take more than an hour of your time and you'll be able to see if there are proper GFIs and clean contacts there at the same time.