Instructions
Get HM03, Surf, if you have not already done so. To get Surf, go to Ecruteak City in the Johto region.
Walk north, past the Pokemon Center. Enter the Dance Theater. Defeat the Pokemon of the five dancers in the building. Talk to the old man in the back of the theater to get Surf.
Go to Cerulean City, located in the Kanto region. Continue east until you leave the city and get to Route 9.
Get into the water when you can walk no further. Surf to the east. If you are seen by the trainers in the water, they challenge you to a battle.
Surf south at the junction, and then get back on land. The large brick building on the shore is the Kanto Power Plant. Go inside the talk to the manager and agree to retrieve the missing part.