As soon as you reach Rocket Town later in the game, on "Disc 2", Cid will join your team.
After the Huge Materia Side Quest you obtain the submarine and can dive under the ocean.
ATTENTION Emerald Weapon is here. He is one of the optional super bosses.
After some diving near Costa del Sol, you will eventually find the sunken Gelnika ship.
ATTENTION the enemies here are very strong i recommend to have high level characters or a ribbon as an accessory. Alternatively use the "no random battle" Cheat with
+
Move to the left and go through the door. There will be up to the north another door. In this room are some chests. The one down the stares in the middle foreground containing Cids Highwind limit item. The Hades Summon Materia lays also on the ground near the broken helicopter.
Like all other characters finish all other Limits and use the item. Achievement done.