Aqua Knight Series by Yukito Kishiro

Inspired by a beautiful woman in armor -- the Aqua Knight -- young Ashika, living near the World's Edge, vows to become a knight himself. When a twisted aristocrat threatens the lighthouse he calls home, the boy gets his chance.

Aqua Knight Ruliya, who wants to save young Ashika from enslavement, is under the spell of the master of a giant submerged castle. Will her one friend in the castle save her from the claws of giant crabs?

No sooner has mad scientist Alcantara landed on Marmundo than he's engaged in a knockdown, drag-out joust with Ruliya. Can Ruliya's years of training and experience save her? With the mysterious ghost-sorcerer returning to complicate the situation, the ultimate battle between this world and the underworld is on!Combining motifs from Spain's Age of Exploration with futuristic technology and bizarre sea creatures, Aqua Knight is a fantasy adventure rich with atmosphere.