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