You need to have surf, but since you beat the 5th Gym it shouldn't be a problem. You surf to the right of Mauville City, or route 118. If you don't have a water pokemon I suggest Zigzagoon, it can learn surf, rock smash, and cut.
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Go back to your dads gym in petalburg
Go to the Pokemon league.
go to your father and fight him to get the fifth badge
Time to go to the Pokemon league and beat the game!
Get your badge and go to the elite four. you must get waterfall.