someone mentioned that the mermaid actually falls in love with a different female character (and love blossoms!!) in this story, not so sure if true but I sire hope so!
Haha, good joke, it's a dead genre, unless you want the character to appear as a mermaid in the first page then transform and stay human to the end with no mermaid lore besides "they need water". Also if you don't want it to be smut, I'm sorry, but you have to dig for so long
In this website you can find
The Story of The Princess and The Little Mermaid (drama, oneshot)
Succuba & Siren
A Monster Wants to Eat Me
Ningen no ane to era kokyu no imoto (incest)
Between the Sea of Clouds (dead translation, bird girl, fish girl)
Mina Soko Nite (low-key psychological horror)
Succuba & Siren (old and Korean)
Aiiro Ningyo (smut with a schoolgirl, grooming, both mermaids)
Good luck finding any of them worthy of your time
Ugh now I REALLY want a GL with a mermaid, anyone has any recommendations?