If its a lake...isn’t it touching the ground already?
As you can see in the diagram from post 15, most of the electrical current returns to the source in the neutral leg. But if an electrical load like a motor has some resistance to ground in the boat, some of that current will return to the source to the source ground first through the water then to the Earth and then to the source's ground.
Since the women were close to the boat or dock, they were a path for the electrical current too. It does NOT take very many milliamps to cause paralysis.