Pokémon S12E09 - Hold the Phione!

Ash and friends arrive in the town where Dawn will enter her next Pokémon Contest. They find out that a lot of people have come to the town in order to see the legendary Pokémon Phione, who lives in the surrounding sea. Legend has it that anyone who sees Phione will have good luck, so Dawn decides to try and catch a glimpse of it.