Water is an insulator but will give electric shock when its not purified in lab

Once in my childhood i had learnt that water does not allow the electricity to pass through it.I was filling an electric water cooler fan tank with water from a pipe.That electric cooler had a mild electric current.I just touched the line of water while filling and got an electric shock.My teacher told me later that it happened due to salts in water.A little knowledge is dangerous.I learnt it that way.