You have to meet cinthia after you beat the veilstone gym shell give you the secretpotion. you give it to the psyducks that are blocking the way above solaceon town. go through the route with a Pokemon that knows defog, youre gonna need it. get through to celestic town and talk to the woman that meets you in front of a cave that has mosaics of dialga and palkia on the entrance. she'll show you the cave and say that she doesn't need surf but you will. l
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Surf is HM #3. :-)
You will get the HM surf after beating the Norman then visit the Wally's father he will give the HM Surf. In the house left of the gym of petalburg city, but you will first need the 5th badge.
Get the HM Surf then teach it to a pokemon.
teach it the HM surf
you have to teach it the hm surf i believe.............