How do you get to cinnabar island from seafoam island in Pokemon leaf green?

seafom island is to the right of cinnabar if you want to cheat a tiny bit fly to pallet town and surf down to reach cinnabar island. But if you want to do it the hard way you need to get the boulders inside of seafoam island to go into the water so you can reach the ladder that leads you to the exit where you can go to cinnabar from.