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.
Electrical current will always attempt to return to its source in order to complete the electrical 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.
Electricity may be introduced to water in a variety of ways, including improperly wired pumps, boats, or lighting, improperly grounded wires, or stray current in the ground. This presence of electrical voltage in water is invisible, so it is crucial that water always be checked for voltage prior to swimming.
Electric Shock Drowning (ESD) occurs when AC voltage present in water passes through your body. Even a low-level of voltage can cause skeletal muscular paralysis, rendering the victim helpless and eventually resulting in drowning.