You have to sleep in the bed in cabin NO. 2. Go to the bed, press "A." The screen will turn black. When it is over, the text will say you have made land in either slateport or lilycove. If it doesn't say that, than sleep in the bed again. Then talk to the sailor guarding the entrance, and he will let you off.
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Leftovers are an item that you can give to Pokemon to hold. In Pokemon Emerald, you can get them from the bins on the SS Tidal.
After you beat the elite four.
Beat the elete four 19 times.
Beat the elite four. Then go to Slateport's port and the ferry is there.
He is on the S.S. Tidal being the captain with his Peeko