Blue Crushed (a.k.a. Baewatch) - S1.E22

Rainbow Dash and Applejack invite Fluttershy to go surfing with them, but she doesn't know how to surf. Her brother Zephyr Breeze suddenly arrives, bragging to Rainbow about being a long-time surfer, but Fluttershy knows for a fact he is lying. When Rainbow and Applejack invite him to show them his surfing skills, he instantly loses much of his confidence but joins them anyway. Out on the waves, Rainbow and Applejack surf with ease, but Zephyr wipes out almost immediately. Back on the shore, he admits he actually doesn't know how to surf, and he begs Rainbow Dash for forgiveness and surfing lessons. Begrudgingly, Rainbow agrees to give Zephyr some pointers.