Go to Junon and start traveling to the underwater reactor. Once wewe are on the long underwater path before wewe actually enter the reactor wewe will get in somewhat frequent battles with a large mashua with a skeleton on the front and it performs an eject songesha with it's paddle. Morph it and wewe get a book of some sort can't remember the exact name.
Then wewe take that book to Kalm and in the, kinda juu 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 wewe the underwater materia. (also give this guy the rose obtained from beating ruby weapon and wewe get a dhahabu chocobo but kwa that time I definitely have one]
