if your talking about the ship in slateport and lilycove that can drive you to the battle fronteir, then deafeat the Elite 4
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You find it in the casino for 4000$ or in the abandoned ship
East of Dewford. Stick to the upper rock and you should find it ^^.
Mauville Casino-4000 coins or Abandoned Ship Storage Room
Pokemon Ruby has Pokemon that Pokemon Sapphire does not and Pokemon Sapphire has Pokemon that Pokemon Ruby does not.
you find them in cabin 1then you have to do some thing it's hard