Pokemon: Season 20, Episode 5

20 - 5: Yo, Ho, Ho! Go, Popplio!

Satoshi helps Suiren train her Ashimari to blow a water balloon large enough to enable undersea travel for people. During a school fishing trip, the class's Ride Pokémon are stolen by Rocket-Dan. However, Ashimari manages to blow a massive balloon to save all the Ride Pokémon from falling on the rocks.