Well, that's as may be, but, my truck has an electric fuel shutoff valve.
Most
modern diesels in truck (and I would think also boats) applications do.
Or how does turning the key to off stop the engine?

Your truck is designed to be started in the same fashion that people are
accustomed to use when starting a car.

A lot of diesel boats don't use the key to start or stop the engine. The key is
turned on to activate the electrical panel and the alternator, but a separate
push button is used to crank the engine. When the engine is shut down, another
push button
activates a solenoid that, yes, does actuate a mechanical fuel shut off
mechanism. Only after the engine comes to a complete halt is the key turned to
"off."

Depriving a gasoline engine of the electricity reuired for the ignition system
would shut down the engine. Unless your diesel needs a constant supply of
electricity to keep the fuel supply flowing,
cutting off electricity will have no effect. As the question was generic in
nature, a choice that applied only in very unique circumstances would not be
the best choice among the answers- even though in those unique circumstances it
would be entirely correct.

My new engine (going in next week, yeah!)
won't even have a key, just a switch. :-)