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First time using this template. It's fun.
My gym would be contained inside a huge facility that is part aquarium, part gym. The entrance is like your typical aquarium, with lots of exhibits and marine tubes and such. This is open to all of the public. At the back, there is a door that leads to a small room with a small diving pool. Only those challenging the gym can enter this room. Since the gym and aquarium are considered separate facilities, the gym music doesn't actually play until you enter this room.
You're given diving gear, and as you dive down you come across a natural underwater system of cavern tunnels. You navigate through these while fighting off diver trainers. You know you're close to the end when you see sunlight. You swim up, and reach a small and secluded beach shore. This is where my gym leader waits.
For winning you get the Scale badge, a badge that resembles a glistening and colorful fish scale, as well as the TM/HM for Surf.
My gym leader's name would be Dylan, which is Welsh for "son of the sea". Though his gym is associated with the aquarium, he's not the owner of the aquarium. The gym and aquarium have agreed to share the same space. Though Dylan loves sea life, he is fascinated with the mythology of merpeople. That's why his team is a bit more mermaid-themed rather than packed with aquarium-esque Pokemon like Sharpedo or Seaking (along with my personal water favorites).