When it comes to safety, you can never be too prepared. Outdoor pools should not be used during a lightning storm because water conducts electricity, and neither should indoor pools. The main reason being that the pools are connected to a larger surface via pipes and wires and a lightning strike anywhere on the network could lead to a shock elsewhere in the same network.
Experts recommend evacuating the pool anytime a storm is within 10 miles of your location. A good way to estimate how far away the storm is is by using the flash-to-bang method. The F-B or flash-to-bang method work by measuring the time from seeing lightning to hearing thunder. For every 5 seconds, the storm is 1 mile away. You should wait thirty minutes after thunder is last heard to reopen the pool.
For more information check out the National Lightning
Safety Institute’s website here.