Growing up as a kid in the 90's, Are You Afraid of the Dark was one of my favourite shows, and I remember this as being possibly the scariest episode.
A chemistry genius with a fear of swimming and a swimmer who has been pushing for a school swim team discover an abandoned pool behind a wall of the school, hidden after a student mysteriously drowned in the pool many years before. As soon they soon learn, the pool is haunted by the restless ghost of one body that was left behind when a cemetery was moved to build it.
When Zeke dumps a container of Methyl Orange into the water to turn the ghost visible (only part of the chemistry lesson we get out of this episode!), he must then overcome his crippling fear of the water to save Clorice from the terrifying being it becomes (a nightmarish-looking red skeletal zombie).