After beating the gym leader Fantina in Hearthrome City in Pokemon Diamond, the player will then be able to teach HM03 Surf to a Pokemon, which is required to proceed in the game. Following the natural gameplay carefully, the player would have obtained the HM03 Surf earlier in the game in Celestic Town.
After exiting the Hearthrome City gym, Cynthia will appear and suggest for the player to head to Canalave City. At this point in the game, HM03 Surf can already be taught to a Pokemon. The player will recall that doing so was not possible after receiving the HM from Cynthia's grandmother in Celestic Town. Once the player has taught a Pokemon to Surf, he or she can then go to Canalave City where the game's next badge is located.