The Scanner is a key item introduced in Generation III.
In Pokémon Ruby Version and Pokémon Sapphire Version and Pokémon Emerald Version, it is found inside the Abandoned Ship, and Dive is required to obtain it. It can be given to Captain Stern in Slateport City in exchange for a Deep Sea Scale and a Deep Sea Tooth.
In Pokémon Omega Ruby and Pokémon Alpha Sapphire, it is found inside Sea Mauville underwater. It can be given to Captern Stern in Slateport City in exchange for a Clear Bell in Omega Ruby or a Tidal Bell in Alpha Sapphire instead.