What is an Electric Shock Drowning (ESD)?

ESD happens when a swimmer comes into contact with electrical current.  The current – in this case, alternating current (AC) – causes skeletal muscular paralysis, lasting for only an instant, but long enough to incapacitate a swimmer allowing him to drown.
Where does the electricity come from?

The electric current “leaks” from boats and docks into the water. It can come from frayed wires, improperly wired systems or an AC grounding system that is damaged or malfunctioning.

What causes people to be electrocuted?

Electrical current will always attempt to return to its source in order to complete the elecrical circuit. Electrical current is resourceful and will find any way to do that, taking the path of least resistance and most conductivity (anything that will help the current move along its path). The way alternating current (AC) searches for its source is the most deadly for humans because it takes only a small amount of AC to disrupt the electrical impulses that control our muscles and nerves.
 
Why does ESD happen in fresh water and not salt water?
We go back to conductivity for the answer. Fresh water is not a good electrical conductor. Because it is not a good conductor, the alternating current looks for something better. A human body in fresh water becomes that something better. The high amount of salt in humans make our bodies far better conductors of electrical current than fresh water.