I would be a Water type Gym Leader with a Gym where you need to hop on a boat/raft and ride on the raft, fighting gym trainers. If the gym trainer is beaten, they will lower or raise the water to allow you to pass to get to me.
My team would be:
Milotic - Moves: Ice Beam, Surf, Rain Dance, Recover - Item: Leftovers - Ability: Cute Charm
Tentacruel - Moves: Surf, Giga Drain, Sludge Bomb, Ice Beam - Item: Assault Vest - Ability: Rain Dish
Politoed - Moves: Mud Bomb, Surf, Scald, Psychic - Item: Damp Rock - Ability: Drizzle
Ludicolo - Moves: Giga Drain, Teeter Dance, Leech Seed, Surf - Item: Big Root - Ability: Rain Dish
Kabutops - Moves: Waterfall, Aerial Ace, Swords Dance, Stone Edge - Item: Life Orb - Ability: Swift Swim
(Ace) Mega Swampert - Moves: Ice Punch, Aqua Tail, Earthquake, Hammer Arm - Item: Swampertite - Ability: Torrent
Obviously my strategy is to set up Rain with Politoed's Drizzle and use Kabutops or Mega Swampert to Sweep. Ludicolo would be used to stall, as well as Milotic. Tentacruel would be my special wall and Swampert would be my physical wall.
I would give out the TM Scald, since it's the only good Water-type TM.