Shinkai Densetsu Meremanoid(1997)

The story takes place in the underwater world of Mer. The army of the kingdom of Bars, until recently peaceful, begins a terrible war against its neighbors: the principality of Moslem and the ancient and glorious kingdom of Agadia. No one can explain how the small kingdom of Bars has reached such a power. In a short time, it manages to challenge much larger and more powerful realms. Meanwhile, two young 'spiritual knights' (Jo and Oz) make a living by killing marine monsters. The population, however, is not very tolerant towards the duo. People fear their powers, seeing them as a potential threat. The pair will unwillingly find themselves fighting the army of Bars, and the forces of darkness that have taken its head...