If you're looking for manga similar to Futari no Otohime, you might like these titles.
Seven years ago, Yura found Nao washed up from the sea. No one knows who he is or where he came from. Her parents took him in, and he's been living as her "little brother" for seven years, but now that is going to change...?
In a peaceful sea world, the world's most respectful Hair-tail fish king ruled so that all the fish tribes/species lived happily. What happens when the prince disagrees on whom he can marry?
It's been years since Marie was saved from the ocean, and has lived on a solitary island ever since. Her first and only friend is Canu, a strange creature from the sea. But she hasn't seen Canu in awhile, and ventures out to find it...