You'll need to teach the Pokemon Surf and go to the water and press A. (If not go in party and select the Pokemon that knows Surf)
u will have to cacth a Pokemon that can learn surf that's the only way
Surf around Lake of Rage
learn surf
its Pokemon heart gold and you surf east of newbark town
when you get HM surf just get in the water and there's only water pokemon.
surf to the right of new bark town.
beat the kimono girls in ekruteak city
When you find the Gold Teeth in the Kanto Safari Zone, give it to the man in the house all the way north from the Safari Zone. He will give you HM Surf for the Gold Teeth.then you will have your HM surf
Gold teeth are used to obtain the HM Surf. The teeth are given to the warden at the end of Safari Zone, and in return he will give you the HM Surf.
Surf there from Olinvine then keep going till you find land
To get mantine in crystal and gold surf around the whirl Islands
finish gyms (all) and go to 1ºtown and use surf