My gym would be located in a domed aquarium. The actual gym would consist of a flooded maze like structure that you must traverse through on the back of one of one of your surfing pokemon or on a raft provided with one of your pokemon. You both must swim through the maze and locate the 4 switches that lift the 4 gates to the gym leader(Me). If surfing your pokemon must stay ahead of a weak current while locating the switch. If on a raft, you and your pokemon must work together paddle to the switches so its all about teamwork. However quite a few of the switches are actually traps that open a trap door to a trainer you must fight. If you are surfing, trying to use dive might me a good idea. Finnaly I would have 6 pokemon:
All the ones with a slash have a 50% chance of fighting either or.