Instructions
Obtain HM3, also known as "Surf," after defeating the Grunt encountered in Ecruteak Theater. Teach this skill to an eligible water-type Pokémon, such as Bulbasaur or Staryu. Continue your journey until you reach Celadon City.
Travel west out of Celadon City toward Route 16, down through Route 17 and east along Route 18. Be wary of the biker gang trainers along these routes. Their Pokemon are leveled between 40 and 60. Have similar or higher leveled Pokemon with you on your journey.
Follow Route 18 into Fuchsia City. Turn to follow the south path out of the town onto Route 20.
Approach the water at Route 20 and you will be prompted to select your Surf-equipped Pokemon. Select "Yes" and jump onto the Pokemon. You can now control the swimming Pokemon just as you would your trainer.
Follow along the Route 20 path, past the Seafoam Islands and Cinnabar Island. The path will turn north to Route 21. Battle any trainers, who mostly hold Level 30 Pokemon, that you encounter along the path until you reach Pallet Town.