This is the story of Aino, a beautiful girl from Karelian-Finnish epic song Kalevala. Her brother offered to marry her on one old mage in exchange for his freedom. But she couldn't accept this fate and believes that life at the bottom of the sea will be the best share for her. Before the wedding Aino leaves her home to meet the sunrise by the sea and perishes in the waves.

But, the spoiler, then she wakes up in the body of the fish ;)

You can read the original text here! - https://en.wikisource.org/wiki/The_.....levala/Rune_IV