Question

Hydro cannon for Gyrados?

Answer 1

Gyarados doesn't learn hydro cannon. I would recomend waterfall, because of his high attack stat:
Gyarados

item: life orb or expert belt
EV's: 252att/ 252 speed/ 4hp
nature: adamant ( +att -sp att)
waterfall
earthquake
ice fang
dragon dance
This set works well in-game and in competetive battling. In HG, if you use two full restores, you can max you're attack and 6-0 lance. I'm not sure what game you are playing, but the set works well

Answer 2

Unfortunately, Gyarados does not learn Hydro Cannon. Only water type starters can learn them:

Blastoise
Feraligatr
Swampert
Empoleon
Samurott

Any Gyarados that uses Hydro Cannon is likely a hacked one. Gyarados does however learn the move, Hydro Pump. However, Hydro Pump is a special based attack. Gyarados's special attack longs in comparison with it's Physical attack power. This means you need a different Physical move if you wish to use one.

As mentioned, Waterfall is a powerful attack that can be used to decimate enemies. Aqua tail provides a higher power than waterfall, but has lower accuracy. Dive is both Powerful and accurate. But it does take a turn to charge. That means enemies can switch before it hits. so it is a matter of preference.

LCB ;}