Location from pokemon-kanto by
The Sensational Sisters' synchronized-swimming gym in Cerulean City — a water-ballet theater that doubles, occasionally, as a legitimate Water-type gym when Misty is present.
Cerulean Gym is not really run as a trainer gym when Misty is away — the Sensational Sisters prefer the water-ballet circuit — and challengers who show up are usually given the Cascade Badge without a real battle. Ash was the first trainer in a while to actually demand a proper match. He got one after Team Rocket attempted to steal the sisters' Pokémon, and won the badge (or, as Misty tells it, was given it after Team Rocket fled). When Misty returns from her Orange Islands journey, she formally assumes the gym leader role and restores the gym's battling reputation.
A sleek glass-and-turquoise pavilion with a full Olympic-sized pool as its arena. Stadium seating, bubble chandeliers, a mermaid-themed backdrop that descends when the sisters perform. The changing rooms are painted pink and cyan.