Pokémon Red and Blue/Route 19-20

'''Head south from Fuchsia City to the beach. Paddle south and west along Routes #19 and #20 until you reach the sunny shored of Seafoam Island. You'll have to make your way through the cave to continue. Alternatively, you can Fly back to Cinnabar Island and approach Seafoam Island from the west along Route #20.'''

Let's go Surfing now
You could do the Seafoam Island dungeon before the Power Plant, but then you'd be depriving yourself of the pleasures of tearing through it with Zapdos' powerful lightning blasts.

The Island can be reached by either heading south from Fuchsia or east from Cinnabar. Yellow players will notice a new house on the beach near Fuchsia: This is Pikachu's Summer Beach, where players with a "Surfing Pikachu" can play a surfing mini-game.

Pikachu's Summer Beach
When, in the course of Pokémon events, a special Pokémon appears on the scene, it's time to party surfer-style. Its origins are shrouded in mystery, legend has it that a special "Surfing Pikachu" exists that can not only Surf but also use Water-type attacks in battle. Surfing Pikachu can't be caught in the game, but there is another way to get him (check the spoiler at the very bottom of the page). If you're a lucky owner of one of these ultra-rare creatures, there's a surprise waiting for you. In Yellow, there's a nondescript house on the shore below Fuchsia City. If you go in with Surfing Pikachu, a kind bro named Dude will ask you if you want to surf. If you accept, you can catch gnarly air in the waves with Pikachu! As the waves scroll by, you'll be given points based on the number of radical stunts you can do off the top of the wave crests. You can head back to the hut to view your highest surfing scores, and even print them out with the Game Boy Printer!