You need waterfall, which can be obtained in sootoplis's cave origin. Or get a goldeen or seaking.
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After beating all 8 Gyms in Pokémon Emerald you need to make your way to Ever Grande City. To access Ever Grande City you need the Waterfall HM. After making it to Ever Grande City you now need to make your way through Victory Road. To challenge the Elite Four, you will need the HMs Strength, Surf and Rock Smash. To make it through the cave, Flash and Waterfall are not needed but will help you go through the cave.
you get it at cave of origin but you have to beat the gym leader of sootopolis city to use it on the waterfall to ever grande city
It is on ever grande Island make sure you have a Pokemon with waterfall.
After this, you surf from sootopolis or thereabouts, to ever grande city in the east. Once there, make your way through victory road, and then the Pokemon league after that
In Ever Grande City.