Hydro Cannon. You can find the move tutor who teaches the move here:
D/P/Pt:Route 228
Hg/Ss: Blackthorn City
B/W: Route 13
They also teach all the Fire starters Blast Burn, the Grass starters Frenzy Plant and all the Water starters Hydro Cannon. They all have to have max happiness for the guy to actually teach it to your Pokemon.
