Less than 40 milliamps (.04) amps at less than 100 volts can put a human heart into cardiac arrest by electric shock. It is the current flow that causes the damage to the human body's natural electrical signals that regulate the heart muscle.
How little voltage and current depends on the conductivity of a particular person and and the duration of the electrical connection. Anyone who has had and electric shock from household level electricity (normally 120 volts at 15 amps) and did not die was saved because they were an incomplete circuit -- wearing tennis shoes on a dry surface while only touching one end of an electrical hot circuit. Instead, they were charged up like a capacitor and yes, the earth ground is a very good electrical conductor.
Electric shock around pumps
When working around pumps with electric motors, dangerous damp conductive surfaces usually prevail. The fatal ingredients tend to be moisture accumulation, wet metal, rust, loose wiring, and carbon tracings caused by electrical arcs and sparks over time. Although people can be shocked by electrostatic generators putting out 100,000 volts or more of static electricity, the current output is negligible.
The primary purpose of electrical grounding is to help provide some safety to humans and other equipment by shunting loose current directly to ground. Sensing devices such as Ground Fault Circuit Interrupters detect a loss or imbalance of electrical current in a circuit, causing it to shut down. GFCI devices are now mandatory on all household circuits such as kitchens, bathrooms, and outside outlets where water can and does accumulate.
A primary use for GFCI's are below surface swimming pool lights where chlorine will eventually corrode the fixture potentially causing electrical leakage into the pool water. Our National Electrical Code is primarily designed around and for human safety. BE THANKFUL IT EXISTS!