Aqua Knight - Kishiro, Yukito

On a small island in the vast ocean of Marmundo lives an old man and a little boy, Arrabarus and Ashika. They are the guardians of a lighthouse, which stores a mystical light they call Niselle. One day, the boy rescues a young knight, lady Ruliya, who was swept off course during a storm. Promising her savior anything as thanks, Ruliya immediately regrets it as Ashika wants to become a knight as her. The little boy decides to become her squire, but first his father challenges her to a duel. While they're fighting, Niselle is stolen by the genius Alcantara, who also kidnaps Ashika. In the battle, the lighthouse is destroyed and by some mysterious connection this kills Arrabarus. Tagmec, knight of the Underworld, emerges from the sea to claim the old man, but not before Ruliya promises to rescue the boy and the light. Tagmec informs her she is embarking on a quest of destiny. He grants her Muertogara, magical blade.
(Source: Wikipedia)

Aqua Knight was published in English by VIZ Media, first in comic book format for a total of 16 issues, split into three parts, from May 2000 to August 2001, and later in tankoubon format, from October 10, 2001 to February 10, 2002.