I didn't know you could do it. Is there not a guard blocking your way or something? I was sure the "underwater" materia was listed in an "unmissables" guide on gamefaqs.com because you need to morph the ship enemy in the underwater tunnel and give the Guide Book to the man in Kalm to get it.
Of course that in itself is wrong because you can morph the ship enemy in the Battle Square at Gold Saucer XD.