The Old Sea Map is no longer available to obtain through Mystery Gift. You will need to use a GameShark or an Action Replay to obtain this item and it is only available for Pokemon Emerald, FireRed, and LeafGreen.
Answer: The Old Sea Map is an event key item needed to access Faraway Island in Pokémon Emerald. It was distributed during the Pokémon Festa event in 2005 in Japan, but unavailable to international players (without cheating). The Old Sea Map is required to access Mew.