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One Island only unlocks once you defeat the Cinnabar Island gym. You can get to the island by boarding a ship. You'll need HM03 Surf. north to the sandy beach. Continue heading north through the tall grass to another sandy area. Keep walking north until you see a cave opening. You'll see "Ember Spa" in the upper left corner of your screen. Follow the path north, then climb a set of stairs, then head east to a man between two waterfalls. He'll give you "HM06 Rock Smash". "Rock Smash" will smash small crumbling boulders in your way.
Go to One Island. Go to the east end of the island on Kindle Road. Surf Enter the cave. Talk to the man.