Go to Junon and start traveling to the underwater reactor. Once आप are on the long underwater path before आप actually enter the reactor आप will get in somewhat frequent battles with a large नाव with a skeleton on the front and it performs an eject हटाइए with it's paddle. Morph it and आप get a book of some sort can't remember the exact name.
Then आप take that book to Kalm and in the, kinda चोटी, शीर्ष right corner there is a house. Go in that house then go up stairs and there will be a guy there. Give him the book, he gives आप the underwater materia. (also give this guy the rose obtained from beating ruby weapon and आप get a सोना chocobo but द्वारा that time I definitely have one]
