Instructions
Complete the Johto region by defeating the Elite Four and the Pokemon League Champion at Indigo Plateau.
Wait for the end credits to roll or restart the game, and you will be back in your room in New Bark Town. Go downstairs and talk to your mom.
Go to see Professor Elm at his lab and he will give you the S.S. Ticket.
Go to Olivine City in the west of Johto and go to the docks. Present your S.S. Ticket at the desk in the docks and you can get on the S.S. Aqua.
Talk to the old man who runs up to you as soon as you get on the ship, and he will ask you to find his missing granddaughter. You'll also be shown your room, where you can sleep to heal your Pokemon.
Go down the stairs in the middle of the top floor of the boat and go all the way to the right. A sailor here won't let you pass, so go back upstairs and and talk to the sailor in the room next to yours. He'll run off downstairs.
Head back downstairs and go all the way to the left. The sailor will let you pass this time. Go up the stairs in the next room and you'll appear in the captain's cabin, where you'll find the old man's granddaughter. She'll run away, but you can find her again at the far right of the lower floor.
Go back to the entrance of the boat on the top floor and the old man will thank you for finding his granddaughter and give you a Metal Coat item. As soon as this mission is complete, the ship will dock at Vermillion City, and you will have arrived in Kanto.