In Pokemon platinum, you can get Surf from an old woman in Celestic Town. First, net a Great Ball and a poketch app by talking to the people residing within the town. The Great Ball is in the Pokemon centre and the app is in the house in the southwest corner of town. You will also find a galactic grunt blocking the cave; defeat him and an old woman will come up to you to show gratitude. Go into the cave and press A, after which the old woman will come into the cave and give you Surf.