Underwater is an Independent Materia in Final Fantasy VII. It removes the 20:00 timer when fighting Emerald Weapon. Underwater does not appear in the original Japanese version of Final Fantasy VII, due to the Emerald Weapon not existing as a boss.
Obtained[]
Underwater is traded from the Kalm traveler with a Guide Book. The Guide Book is morphed from a Ghost Ship in the Underwater Reactor or the Battle Square. Only one Underwater Materia can be obtained, but only one is needed.
Use[]
Underwater removes the 20:00 timer from the Emerald Weapon fight, though it is possible to beat Emerald Weapon within the limit. This means it should only be used if the player needs the extra time. The Materia can be given to any party member, however, Barret's Missing Score weapon treats the Underwater Materia as if it has 0AP regardless of how much AP it holds, meaning it should not be given to him as it will not increase his damage.
The Underwater Materia gains AP, but will never level up or grow into another one. Underwater Materia can be mastered, but only if it gains exactly 16,777,215 AP.
Materia priority system[]
Because of how mastering Materia works, the player needs to be careful when their Materia inventory is full. When the maximum amount of 200 Materia are already in the inventory, but a Materia is mastered in battle, the game will delete a pre-existing Materia from the inventory to make room for the newly-created one. The game uses a priority system to decide which Materia pieces to delete, with the lowest priority being the first to be deleted. The game gives no indication or warning that this has happened, so the player will only notice if they check their Materia menu.
Nearly all low priority Materia are ones that can be bought at shops, such as Fire, Ice, Lightning, Restore, etc. The exception to this is Underwater, which—despite only a single copy existing—is the lowest priority Materia and will be the first to be deleted. If it is deleted, there is no way to obtain a new copy. Aside from being unable to use it for the Emerald Weapon fight if it has not been fought yet, this also locks the player out of achievements given for obtaining all Materia in later versions of the game. Because of this, if the player is getting close to their 200 Materia limit, it is recommended to keep Underwater stored on the equipment of a character who is not in the active party, as the game will not delete equipped Materia.
Behind the scenes[]
Early in development, this materia may have been called Aqualung. An "Aqualung materia" appears in some of the earliest content for the game, and can be hacked into the original Japanese PS version of Final Fantasy VII. It appears that at one point there was a plan for an underwater dungeon where the player would need the Aqualung materia to enter or they would drown. When the dungeon idea was scrapped, only the name of the unused materia was left in the game data, which was later repurposed into the Underwater Materia in the overseas and International Version releases of Final Fantasy VII.
There is a hint to the Underwater Materia's location in the game versions that include it: Shake at Wutai Village's pagoda says that there is a rare materia held by a monster in an underwater tunnel, but one "needs to use their brains to get it". In the final version, the Materia is actually held by the Kalm Traveler and not the undersea monster, through a trade of the monster's Morph item.