Hyrdo Cannon is not a good move. It has a recharge turn, 90% accuracy and only 5 PP. The recharge turn is the most damaging, since Empoleon will have to take damage on that turn. If you want a powerful water move, use Hydro Pump instead.
Flash Cannon is not needed. It's not as if Ice Pokemon will do much damage to you, so just use your STAB water move, and as for Rock Pokemon - they're weak to water anyway.
And your water moves already cover ground weaknesses.
Considering this is in-game, I would give your Empoleon this set:
Empoleon @ Leftovers
- Surf/ Scald
- Ice Beam
- Aqua Jet
This is pretty much the standard set for most water starters; I used it for my Samurott in W2, and this set is pretty useful in getting you around and battling.
~ Waterfall & Surf - STAB HM moves, they will be essential to getting around and they're not a waste move slots on Empoleon since it will be able to use them to good effect in battle. I'm also suggesting Surf > Hydro Pump because it's more useful in game and has 100% accuracy
~ Aqua Jet - priority STAB, clears any stragglers without you taking damage
~ Ice Beam - coverage; it will cover your Grass liability
For a competitive set:
Empoleon @ Life Orb
EVs: 252 Speed, 252 Sp. Atk, 4 HP
Nature: Modest/ Timid
- Ice Beam
- Grass Knot
Modest/ Timid are the preferred natures for Empoleon, but I know you don't have much control over natures in game, so you need to work with what you have.