Manga written and drawn by Yukito Kishiro, the creator of Gunnm (also known as Battle Angel Alita). Despite this, Aqua Knight is almost completely unlike Gunnm. Gunnm is a dark and dystopian post-apocalyptic cyberpunk story. Aqua Knight is a light-hearted fantasy tale. Gunnm takes place in a scrapyard in the middle of a desert. Aqua Knight takes place on a world covered almost entirely by ocean. The art styles are quite different as well. Kishiro is one of the more versatile manga artists out there, able to do everything from Gunnm's hyper-detailed artwork, to Ashen Victor (which resembled an American comic book more than it did a Japanese manga), to the simplified (but still good) art in Aqua Knight.
As far as the story goes, as mentioned above, it's pretty much a fantasy adventure story, with strong comedic elements, and hints of an interesting plot (which many such stories miss). The main characters include a young boy who lives with his father on a remote island (Somewhat remeniscent of Hayao Miyazaki's classic anime Future Boy Conan in which the boy lived with his grandfather on a similarly remote island. He encounters a (rather attractive young female) knight, who wears armor resembling a cross between a giant robot and an old-fashioned diving suit, and who rides around on a trained orca. A villain shows up, and needless to say, a variety of adventures ensues. The story is not finished yet (and Kishiro has put it on hold to work on more Gunnm again--YAY!), but from what I've read, it resembles a strange cross between Future Boy Conan and the anime Photon (which also features a young boy who encounters an exotic woman, and is chased by a comically vain and egotistical villain).
There is a lot of typical gag manga humor, including the infamous coil of feces (how they manage to get such perfectly shaped piles of shit, I'll never know) so if you don't like such, you might want to steer clear of this. Also, the boy's loincloth is much too short to do an effective job of covering that which loincloths are meant to cover, and the kid gets more fanservice scenes than most sailor-suited schoolgirls. Still, it's worth a read if you don't mind things like that, and it's got Zuma the Blue Cat, possibly one of the coolest characters ever. Then again, I have a thing for cats.