Where do you look for the move surf in Pokemon diamond?
Cynthia's grandmother gives you it after the bomb scare, which is after the Psyducks-ness. You need the SecretPotion to cure the Psyducks. You get that from Cynthia after you battle the Galactic Member you're supposed to stalk from Pastoria (The location you get the the SecretPotion is at the Valor Lakefront).
First of all you need to have a DS that can play gameboy games. Then you need to have gotten Surf and the National Pokedex. With emerald in your GBA game slot and diamond in the DS game slot you can transfer 6 pokemon from your PC in emerald into diamond from the start screen. Once pokemon have been transfered, go to the Pal Park in route 221. You can "recatch" your pokemon with the…
In Pokemon Diamond, Pearl, and Platinum you can find Tentacool in almost any large body of water. Once you get the move Surf from the Pastoria Gym go back to Sandgem town. You have to teach a Pokemon surf first, pretty much all Water type Pokemon can learn it. Then go down to the beach and use surf in the water. Swim around a bit and you're bound to run into a Tentacool.