Remove Hydro Pump
This is the answer you probably don't expect but here's why. Hydro Canon is less reliable than Surf and you have to recharge the next turn. A turn that could decide a tight situation. Keep Surf as you seem more special at the moment, although Swampert is more physical based.
Remove Hydro-Pump and replace with maybe Hammer Arm or Brick Break. If you still want to go special Focus Blast will do.
If you still want to replace Surf I'd suggest Sludge Wave to cover for the quadruple grass weakness. You probably won't be able to before the E4 but try get Ice Punch on youe Swampert. Stone-Edge Or Earthquake would do OK as well.
A Moveset you might want to try out later :
-Earthquake (STAB Good power and accuracy)
-Waterfall (STAB Good power Accuracy and plus flinch Chance)
-Ice Punch (Coverage for that Grass Weakness)
-Hammer Arm/Stone Edge (General Filler move)