I've seen this happen with batteries- they'll get an internal short and you're done. And when yopu hook jumpers to them, you can smoke the cables and the other vehicle you're using to jump. That would apply to a second battery. Have each battery checked independently.
I would also question the switch. I always assume the worst when it comes to unknown workmanship. And I'd be very suspicious why the boat only had one battery.
I would also question the switch. I always assume the worst when it comes to unknown workmanship. And I'd be very suspicious why the boat only had one battery.