Here's why:
* Hydro Cannon is a TM move. While some Pokemon can learn TMs through leveling up, Feraligatr doesn't learn Hydro Cannon in this way.
* Hydro Cannon TM is not available in SoulSilver. This TM is not available in the game's normal progression.
How to get Hydro Cannon in SoulSilver:
* You'll need to trade for it. This means trading with someone who has a Pokemon holding the Hydro Cannon TM.
* Using a cheat device. This allows you to modify the game to give Feraligatr the move, but it goes against the spirit of the game.
Alternatives to Hydro Cannon for Feraligatr:
* Surf: A powerful Water-type move available early in the game.
* Hydro Pump: A strong Water-type move learned by Feraligatr at level 44.
* Ice Beam: A great move for hitting Dragon-type Pokemon, which are weak to Ice.
While Hydro Cannon is a powerful move, it's not essential for Feraligatr to be effective in Pokemon SoulSilver. There are other strong options available to you.