Successfully complete the game to unlock Surf for Pikachu. To teach your Pikachu Surf, unlock Round 2. Then, get up to the Masterball setting on either the Poké Cup or Prime Cup (in Round 2). Complete the Masterball setting with Pikachu as one of your active Pokémon each round (he must be one of the three you choose to battle with). If you lose, you cannot continue (it must be a pure win and the Pikachu may not be part of a registered team or a "Rental"). If you succeed, the game will congratulate you and ask if you would like to replace one of Pikachu's moves with Surf. Select "Yes", then choose the move to be forgotten. The Pikachu will now pull out a surfboard and ride the wave whenever Surf is used.