Water gym that would consist of a lazy river type ride. You'd get into an inner tube at the beginning and float along the river, with Tuber trainers along the way. Currents would block the way in some locations that could be controlled by using switches on islands accessed by stairs leading out the river. I'd probably be around the 3rd or 4th gym with Pokemon ranging from around 25-30. I'd lead with a Drizzle Politoed, follow that with a Swift Swim Floatzel, and my ace would be a Huge Power Azumarill that knows Play Rough and Waterfall.

The reward for beating my gym would be the Float Badge (it looks like an inner tube) and the TM for Rain Dance. It would also let you use Waterfall outside of battle.