These are all GLs:
https://www.mangago.me/read-manga/a_monster_wants_to_eat_me/ (manga)
https://www.mangago.me/read-manga/mina_soko_nite/ (manga)
https://www.mangago.me/read-manga/between_the_sea_of_clouds/ (manhua)
https://www.mangago.me/read-manga/sea_of_june/ (b&w manhwa)
https://www.mangago.me/read-manga/seeking_a_beauty_in_the_east_sea/ (manhua)
https://www.mangago.me/read-manga/the_sea_painted_by_the_moonlight/ (manhwa)
https://www.mangago.me/read-manga/the_wind_spell/ (manhua)
https://www.mangago.me/read-manga/toward_the_blue_athrea_scans/ (manhwa; apparently she is a selkie, which is kind of similar to a mermaid)
https://www.mangago.me/read-manga/the_siren_s_light/ (western webtoon; she's a siren, which is also kind of similar to mermaids ig)
https://www.mangago.me/read-manga/fox_and_elixir_athrea_scans/ (b&w manhwa; not revealed yet, but the FL is some kind of sea creature possibly)
https://www.mangago.me/read-manga/dark_mermaid/ (manhwa; bullying, sexual harassment and assault, attempted sexual assault, blackmail [none of these happen between MC and FL])
https://www.mangago.me/read-manga/the_story_of_the_princess_and_the_little_mermaid/ (manga; one-shot)
https://www.mangago.me/read-manga/the_voice_of_a_mermaid/ (manga; one-shot)
https://www.mangago.me/read-manga/transient_bubble/ (manga; one-shot)
https://www.mangago.me/read-manga/the_little_mermaid/ (manga; one-shot)
https://www.mangago.me/read-manga/the_first_love_of_the_sushi_restaurant_owner_is_a_mermaid/
and Augustus in the human world (please read this on webtoon to support creator)
Mermaid list enjoy
https://www.mangago.me/home/mangalist/1741466/
Mermaid/merman manhwa. Anything you know. Bl,gl or straight