Ah, that’s almost the same as one of my favourite folk songs…

Great Big Sea’s The Mermaid, about a mermaid and her sister, the reverse mermaid:

But then one day, she swam away
So I sang to the clams and the whales
“Oh, how I miss her seaweed hair
And the silver shine of her scales”
But then her sister, she swam by
And set my heart a-whirl
Cause her upper part was an ugly fish
But her bottom part was a girl

Yes her hair was green as seaweed
Her skin was blue and pale
Her legs they are a work of art
I loved that girl with all my heart
And I don’t give a damn about the upper part
'cause that’s how I get my tail

https://www.youtube.com/watch?v=Mp725C0_qyQ