Castle Swimmer is the only one I can think of. It's also available on Webtoon app with more chapters.
Enjoy!
https://www.mangago.me/read-manga/wadatsumi_no_yometori/
https://www.mangago.me/read-manga/itoshiki_ningyo/
https://www.mangago.me/read-manga/wave_nail/
https://www.mangago.me/read-manga/mermaid_s_egg/
https://www.mangago.me/read-manga/samezoku_e_no_sasagemono/
https://www.mangago.me/read-manga/do_you_wanna_touch_my_tail/
https://www.mangago.me/read-manga/wild_fish/
https://www.mangago.me/read-manga/orenchi_no_furo_jijou/
https://www.mangago.me/read-manga/koi_uta/
https://www.mangago.me/read-manga/rain_shower/
https://www.mangago.me/read-manga/gone_with_the_bubbles/
https://www.mangago.me/read-manga/i_suspect_the_movie_emperor_is_luring_me/
https://www.mangago.me/read-manga/mermaid_s_love/
https://www.mangago.me/read-manga/junketsu_ningyo/
Mermaid bl rec excpet desharow mermaid and the falling merman pleasee