Taking a Shower During a Thunderstorm Can be Dangerous.
Yes, although it sounds like an urban legend or old wives’ tale, taking a shower during Thunderstorm (a storm with thunder and lightning and typically also heavy rain or hail) can be dangerous; there's the possibility that the lightning storm can electrocute you.
How It Can Happen
Generally, when lightning hits a house or a building, its inhabitants are pretty well protected, because of the metal framing, ductwork and plumbing that are better conductors than a human. Electricity from a lightning strike shall get conducted through one of these things and then dissipate into the ground, because it follows the path of least resistance to the ground. In other words, during a thunderstorm a protected building acts like a metal cage and there won't be any risk -- unless you touch something connected to plumbing, electrical wiring or any other conducting path.
When you take a shower or a bath, or even start washing your dishes at the kitchen sink, there's a possibility that you might become the better conductor for electricity to flow through. This is possible when the water flowing through your pipes contains impurities that help conduct the current and that your body’s resistance to electricity is cut significantly when you are wet. In this way, when lightning strikes while you are in contact with a pipe, faucet or water flowing through it, it is possible that the current can find its way to shock you, and that will not be pleasant. MythBusters Jamie Hyneman and Adam Savage constructed a makeshift house with complete grounded plumbing to test this weather alert. When they doused it with simulated lightning in an electricity testing facility, along with a ballistics gel dummy with roughly the same electrical conductivity as the human body, it caused fire! In the real world, the odds of being harmed this way are extremely minute, but it is not unheard of.
Lightning Shock Incidents
A UK woman Josephine Martine was blown out of her bath tub by a lightning bolt after the lightning traveled through the pipes and into the shower head she was touching. According to Ron Holle, a former meteorologist with the National Oceanic and Atmospheric Administration (NOAA), who tracks lightning injuries, 10 to 20 people in the United States are shocked annually while bathing, using faucets or handling appliances during thunderstorms.
Be Safe Than Sorry
When lightning has struck this way during storms, people have been shocked and even killed washing dishes, doing laundry and sitting in bathtubs. So it is better to avoid any activities involving plumbing during a thunderstorm. As suggested in Weather.com, avoid contact with electronic devices, including telephones (though cordless phones and cell phones are safe), and don't take a bath or shower during a thunderstorm. The National Weather Service warns it is safe to shower only once thunderstorms have passed you by.