That would work,
however the kill switch that Matt was referring to is not an actual physical switch (some do combined both), it is an electronic engine kill switch that is connected to the 3rd channel and to the coil.
if the voltage drops too low (fail safe kicks in), same for radio loss, also engine will stop running if for some reason the battery pack becomes disconnected (total loss of power), and you can turn the engine off using the third channel on the radio.
Here is a link where a fair amount of people buy these kill switches, the one I will link you too is the Kit #2, they have 3 different V2 kits and made for 2, 3, and 4 channel radios.
Regardless if you have a kill switch or not, you must setup your system to kill your engine using the trigger on your radio by braking.
keep in mind that your radio comes with a built-in fail safe that will close your carb in case of radio loss (signal) or low voltage, but NOT total power loss, that's where the added electronic kill switch takes over.
you would be looking at kit #2 for boats ( you pick what version you need)
http://www.killerrc.com/products/gas_killswitch.php
Dan.