This is a common misconception. It's been debunked many times. It stays around because it sounds interesting.

https://www.finishing.com/156/65.shtml

What is rain water? It is water distilled by the sun. Are there electrolytes in your spring water that you can't get by eating food... no.

Can you get enough electrolytes just from normal water (e.g. Total Dissolved Solids of 200ppm, not Gatorade) without food that will prevent you from dying of an electrolyte imbalance... no. Eventually, only drinking water will kill you for a variety of reasons (starvation), but you have so much salt already in your body that it will take weeks to kill you.