I ran a mercury kill switch from Rex marine. It's a really good safety feature to have even if it's a lake boat. It's really simple to wire in also, you run it inline with your ignition wire so when the cap is pulled it stops the spark to your motor
Typically on a battery ignition, one would place the switch in-line in the power lead from the ignition switch to the ignition box or coil. Magneto styles ground out the mags, etc. I have used both the Mercury style and double ball style with reasonable results. On the double ball, over time you may need to clean the contacts and/or make sure the plastic base does not warp or crack as that may affect the contact pressure and continuity on the switch.
Definately should be a "have to have one" item on your list.