Off Means Off
Here's how it should be wired: #1 battery's positive goes to one point on the switch. #2 battery's positive goes to another point. The third point on the switch is the output, and that goes to your engine and the electrical panel. Both batteries' negative leads are connected together to the boat's common ground. In position 1, the #1 battery is connected to the output and is the sole source of power to the boat. #2 is completely isolated. Similarly, in position 2, the #2 battery is the sole source and #1 is isolated. In Both, the two positive leads are connected together in the switch and the two batteries in parallel provide the power to the output lead. Off means Off. Both batteries are completely isolated from the engine and the other electrical systems. Nothing runs, nothing starts (Well, with one small exception: The #1 battery has a separate (fused) connection to the bilge pump and its float switch so the boat can try to save itself while you're gone). Daniel is right. Your boat is wired wrong. You need to get it looked at by someone who knows his way around boat electrical systems. Think about it - if this very basic wiring is wrong, what else might be wrong? And not just incorrect wiring, maybe also bad wiring (as in sparks). Please, get it looked at soon!
