You don't. That is only in Emerald version.
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go to the marsh and just run in mud or surf in water
You go to where the girl (Jasmine) is under the Gym on the beach and she will give you Waterfall (the HM obviously. You surf on the water behind her and when you get to the waterfall use waterfall, then go through Victory Road, and Then face the Pokemon League.
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You must use Surf. Which can be given to a water Pokemon. Go to the edge of the lake (Like your fishing) and press "A" it will say: The water is a deep blue color, would you like to surf on it? Say yes. But this will only come up if you have a water Pokemon with surf on you.
The "serfboard" allows you to go across shallow water Source: I've played it