Soul Blazer is a video game developed by Quintet. The world has been sealed away by dark forces. Its only savior is the avatar of The Master, sent forth to restore the world to the way it ought to be.
The third location that the avatar is sent to is an underwater city populated by mermaids. Or it was populated by mermaids until they were all sealed away in monster nests. As the avatar destroys the monsters he slowly recovers the mermaids, their buildings, and the dolphins and fish that also live in the city. Finally, after defeating the area boss, he restores their queen.