Ok, i'm guessing it's ghost ship, guidebook, or underwater materia... since they're all related very closely and fit... since you use transform on ghost ship to get the guidebook which you trade for underwater materia

I may well be just what you need
I'd love to bring you up to speed
Curiosity's killing the cat you say?
(Cait Sith comes with transform materia)

Then kindly turn the other way!
(manipulate materia?)

The price is high no matter what
But come and get it while it's hot
Else hold on until you are nine;
(not sure if you have cid when you first use submarine now that i think about it...)

Fortitude equally divine
Shan't blame you if you wish to wait
But look, eighteen plus fifty-eight?
When time's a factor you will see
You will come running back for me
(Time is a factor when you don't have underwater materia when fighting emerald weapon...because you can't breath underwater XD)