Beat the weapons and morph a ghost ship to get the guide book and give it to the man in Kalm to get a underwater materia, you will also get the earthharp for beating emerald weapon which turns into master magic materia,master summon materia and master command materia when you take it to the man in Kalm. and beating ruby gets you the desert rosse which turns into a golden chocobo when you take it to the Man at kalm.