How do you get surf on pokemone Pokemon FireRed?

In order to get Surf on Pokemon FireRed you must go to the Pokemon Mansion on Cinnabar Island. In this mansion you need to find the Pokemon researcher on the far left of the room. She will give you the Hidden Machine (HM) "Surf" upon receiving the Pokedex from you. Now that you have Surf, you can go onto any body of water and use Surf to cross the water.