Good to know it is working for you :-)
Brewgyver has it right - the offset terminal is for the internal illumination of the switch because in boats - unlike in cars - you cannot "ground through frame", i.e. juice has no way to get back to the battery unless you provide a wire for it to do so.
I can't tell from your pics, but if the round little gizmo on the left side of the front of the switch is a little push-button, then it will be a circuit breaker or a fuse holder, and will give you the fusing people have been talking about.
Do you have a "universal meter"? If you don't, I highly recommend that you buy one. they are mebbe thirty bux at places like "RadioShack" and they are absolutely indispensable for diagnosing electrical
problems in existing wiring and for installing new wiring correctly. Once you get the hang of using one, a multimeter is also indispensable for uncinfusing yourself ;-)
I am attaching a quick'n'dirty perspective sketch of what the switch should look like from the back and indicated where the wires go.