Ah-ha!
É a trilogia Aquasilva , de Anselm Audley:
Da entrada da Wikipedia:
Heresy (2001) is set within the stormy waterworld of Aquasilva. Aquasilva is controlled by the Domain, a religious power dedicated to the element fire. The Domain, however, is confronted by many forces of change. One of these unknowing change agents is Cathan, son of the count from Lepidor. Upon discovering iron on their territory, Cathan leaves his home to inform his father of this important find. During his journey Cathan stumbles upon a plot to unleash a new age of fundamentalism. New friends and new powers enable Cathan to confront this extremism.