Yeah, some folks think it's OK indoor but I had to forced them to leave

[theking]

..in the past as a Lifeguard because lightning can travel so even an indoor pool can be affected by lightnings:

Why it's not safe to shower, wash your hands or do laundry during thunderstorms

This can happen because metal and water are very good conductors of electricity. So if your home gets struck, the charge from the lightning may travel into your home and come into contact with you if you're using metal appliances or plumbing.