How to Get a Scanner in Pokemon Sapphire

Travel to Dewford town and head down to the beach. Surf east until you come to the Abandoned Ship.

Enter the door into the Abandoned Ship, then travel around the locked cabins and into the back of the ship on the left side. You will find a Pokeball with a "Harbor Mail" item in it, as well as a set of stairs leading down.

Descend the stairs. You will immediately be accosted by a sailor. Defeat his Wailmer and Machoke and continue onwards.

Enter the cabin at the bottom-left corner to obtain an Escape Rope item. Exit the cabin, then enter the broken door to obtain a Dive Ball. Exit this cabin and finally go into the middle cabin to find a Dive spot.

Dive into the water, then go as far left as you can underwater. You will see a hole in the wall above you. Go through this hole, then surface.

Go into the door all the way to the south and east, then walk to the northeast corner. Walk one space south and press "A" to find the key for Room 1. Open the Pokeball, then exit the room.

Go all the way south and west and enter the door you find there. Walk to the square immediately northeast of the hole in the floor and press "A" to obtain the Room 4 key. Open the Pokeball here, then exit.

Walk north then west to the locked door. Open and enter the door, then search the trash can to obtain the key to Room 6. Exit the room.

Go north and east to the next locked door. Enter the door and grab the Pokeball, then walk two spaces south and two spaces west of the southern-most trash can. Press "A" to find the last room key.

Exit the room and walk all the way south to the locked door in the middle of the southern wall. Open and enter the door and pick up the scanner.