That depends on the game. In Diamond/Pearl/Platinum, there is a move tutor on Route 228 that will do the trick, assuming your Feraligatr's happiness is high enough.
I'm not sure if it's possible before HGSS.
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The only Pokemon that can learn Hydro Cannon are: Blastoise, Feraligatr, Marshtomp, and Empolion.
it does not learn hydro cannon you have to get the tm hydro cannon
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.
Feraligatr can learn Hydro Pump at level 58 in Pokemon Crystal.
swampert can not learn hydro cannon in R/E/S it can only learn hydro cannon in Pokemon pearl/platinum/diamond SEE YA