I'm assuming you're talking about a kill switch with a lanyard attached to your wrist or life jacket. You fall out - the lanyard disconnects from the switch - the motor shuts down?
If so, how do you turn off the motor now? Just add a kill switch that performs the same function as the switch that turns off the motor now. I used one like this for a mud motor I built several years ago: https://www.ebay.com/itm/Motorcycle-Atv-Dirt-Bike-Handle-Bar-Mount-Tether-Lanyard-Engine-Kill-Switch/192890659165?epid=28002731586&hash=item2ce92de15d:g:09QAAOSwk1hctOKR:sc:USPSFirstClass!08302!US!-1
If your existing kill switch works by closing a circuit, wire the new kill switch in parallel with the existing switch. If your existing kill switch works by opening a circuit, wire the new kill switch in series with the existing switch.