From The Mega Man Universe
Aqua Man was originally a robot created for water quality management at a waterworks bureau. His entire mid-section is a tank that he can store water in.
After being modified for battle by Dr. Wily, Aqua Man uses his stored water to attack. He's able to manipulate water in a way that makes it less liquid. His primary attack is the Water Balloon, wherein he fires spheres of water.
Aqua Man generally has a cheerful demeanor and he loves jokes - especially puns and dirty jokes. He also finds farting humorous. Despite being a comical person, though, he's self-conscious about his figure (which he attributes to water weight). He also dislikes serious situations.
Mega Man 8 Encounter
No doubt about it, Aqua Man is tough with just the Mega Buster, but it's the only weapon you have to use against him right now, although Flash Bombs do seem to momentarily interrupt his attacks. For attacking Aqua Man, having the High Speed Charge and the Arrow Shot enhancements do seem to help out a bit. Your greatest advantage lies in the fact that Aqua Man tends to stay in one place when he's executing his rather easy to avoid attacks, which consist of "Water Cannon" (he shoots a jet of water that will randomly change path, though it's easy to find a crevice to hide in), and "Water Balloon" (he shoots a stream of water bubbles that follow you across the room - slide or run to avoid it), and a lot of jumping (slide under him to avoid taking crash damage). If you need to, use Rush Health in the middle to refill yourself if you find Aqua Man beating you up too badly. Just don't let up, and you should beat him.