Puka is a Pikachu owned by Victor, a surfer, that appeared in The Pi-Kahuna. It is unique in that it can predict when swells perfect for surfing will form. This power seems to have also made its eyes blue. Victor had rescued Puka from the sea, and had taken him in. After meeting Ash and co., it used its power to help Victor ride the Humunga Dunga, a huge wave that, previously, only one person surfed.
In the games
The second reference to a surfing Pikachu came from Pokémon Yellow, where there was a mini-game called Pikachu's Beach in the house on Route 19. The mini-game can only be played if the player has a Pikachu with the move Surf with him.