Does your boat have Electronic ignition? if so it's not used anymore, it was for the dropping resistor and coil, the 12v bypass. If you have a Points igintion, you need to hook it to the soliniod wire, and use a DIODE to prevent reverse current flow to the starter soliniod, once started.
Sleek
If you have the (I) terminal this is where the wire goes. The above is if you don't have more than two terminals
http://www.chevelles.com/techref/tt4a.gif
Or you can do this is you still run points and din't have the ignition terminal
http://www.chevelles.com/techref/tt4b.gif