How do you get the Pokemon gym leader humilau city in black 2?

In Pokémon Black 2, to challenge the Humilau City Gym Leader, Marlon, you need to first defeat the Elite Four and become the Champion. After that, you can access Humilau City by traveling through Route 22 and crossing the water. Once in the city, enter the gym, which features a water-themed puzzle involving swimming to reach Marlon. Defeating him will earn you the Wave Badge and the ability to use Surf outside of battle.