I think you are misunderstanding the nature of electrical grounding. If they had laid wires at the bottom of the marsh, then yes it likely would have grounded. However if you float the wires to the surface then the water and the nonconductive particles in the water act as resistors. When the soldier enters the area they act as a conductor, acting as the path of least resistance from the source to the ground.
Why?
I think you are misunderstanding the nature of electrical grounding. If they had laid wires at the bottom of the marsh, then yes it likely would have grounded. However if you float the wires to the surface then the water and the nonconductive particles in the water act as resistors. When the soldier enters the area they act as a conductor, acting as the path of least resistance from the source to the ground.
It’s pretty basic physics.