go to the dessert and take your blastoise/empoleon/feraligatr/swampert with you (they need to love you very much however) and get in one of the houses and talk to the old man
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you cant only in Pokemon pearl,diamond and platinum
swampert can not learn hydro cannon in R/E/S it can only learn hydro cannon in Pokemon pearl/platinum/diamond SEE YA
The only Pokemon that can learn Hydro Cannon are: Blastoise, Feraligatr, Marshtomp, and Empolion.
Why do you need a cheat code for Hydro Cannon????????????????????????????? If you have a Blastoise, Swampert, Feraligatr, or Empoleon you just have to bond with it and you can learn the move WITHOUT messing up your game with hacks.
go to the old mans house on route 228 he will teach starter Pokemon powerful moves