Underwater materia?

It can have more ap than KotR
It helps you fight Emerald Weapon
It does no damage at all

Not a command; VERY helpful against the most powerful boss, Emerald Weapon; Not weapon or armor; Exceeds KotR in AP; absolutely 0 strength on its own, can't even be used to power up Barret's Missing Score.

And if this is it, thank you guardian for pointing out AP