You have received three answers where the more interesting one if from JC. I will not put in question his word or the one of his neighbour's daughter. Many strange things can happen with the lightning. But ...
First question is: In your country or region, is it common to have the "earth" of your power supply connected to the house piping system.
Next question: In your area, do you have aerial power lines, or under the ground?
Of course, if lightning strikes a powerline over the ground and it leads to your house, and the earth of it is the piping of the house, then there is a remote chance that a part of the bolt jumps from an electrical output to the piping system and it could be via the water of the shower.
But you must understand that if lightning manages to shortcut between the clouds and the earth, it takes also the shortest route anywhere near your house. Unless lead by a wiring system, lightning never "enters" a house. Why would it?
Please, believe me; lightning is not as dangerous as many think. First, check the wiring in your house and if it is not part of an external electrical circuit, take a shower, watch TV or use a cell phone; nothing will happen. If you think piping and wiring might be influenced by a lightning strike, ask professional advice to secure your home. It is not only about you taking a shower but the danger of fire and the cost of the insurance.