Ground Fault: A ground fault occurs when electricity takes an unplanned path to ground.
If the grounding conductor aboard a boat connected to shore power is broken and a ground fault occurs, current may find its way onto the prop and from there will return through the water to the shoreside grounding electrode. If a person is in the energized water, they can become paralyzed and drown.
The boat is connected to shore power and the electrical system is operating correctly.
In the image above, a ground fault has occurred in the microwave, but the grounding conductor is intact and only a small amount of current has found its way into the water from the boat propeller. A person in the water may feel only a tingling sensation.
The grounding conductor is broken and now the water carries all of the fault current. A person in the water may be paralyzed and drown.