From Bulbapedia, the community-driven Pokémon encyclopedia.
The Mikan Gym (Japanese: ナツカンジム Natsukan Gym) is the official Gym of Mikan Island. The Gym Leader is Cissy. Trainers who defeat her receive the Coral-Eye Badge.
The Mikan Gym made its only anime appearance in Fit to be Tide.
The Mikan Gym is a barn that has been converted into a beach house. It is shaped like an orange soda can on its side. Inside is a pool that converts to a standard battlefield and back. The beach near the Gym is also buoyed off for Pokémon Surfing races.
While the interior of the Gym is built for a Pokémon battle in the water much like the ones in Cerulean City, Sootopolis City and Pastoria City, it's actually used so that all Water Pokémon have an equal standing while they compete in the accuracy challenge. This was revealed through a section in the wall by the pool where numerous targets had been placed, with the winning Pokémon being decided by who shot the most targets with their attacks.
The Mikan Gym's final challenge takes place outside with a surf race. The race involves both Trainers to surf on top of a Pokémon out into the ocean where they must go around a designated buoy and come back. A natural hazard shown in the anime is the potential for both the Pokémon and their rider to be knocked around by the waves, although a skilled surfer may be able to use the waves to their advantage.