What piece of equipment on boat is most important in preventing propeller strike injuries?

ignition safety switch lanyard

ETA:

AKA: the "Kill Switch". Better than that is a good competent knowledge of boat handling and situational awareness.

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While the above may kill the engine if the driver falls overboard, it would have no effect for when the driver runs over a person or an animal in the water. To prevent propeller injury , the propeller needs to be encased in a shroud or guard so that large objects can not be drawn into the blades. In my view all pleasure craft with exposed propellers should be fitted with these but people do not do so because it increases drag.