storm.

"It must be emphasized that there has never, ever been one reported hospitalization of an indoor pool patron during an electrical storm. By way of comparison, since 1990 the Consumer Product Safety Commission, has reported scores of electrocutions and shocks at indoor swimming pools because of improper use of hair driers, power washers, power equipment, underwater lights and radios and other faulty electrical equipment, but NONE have been caused by electrical storms."

http://www.sportrisk.com/2011/08/the-myth-around-closing-indoor-pools-when-theres-lightning/