It's umi no soko if anyone asks. Don't read the one uploaded here in mangago cus the chapter pages are in the wrong order. Would ruin the experience. Rape warning too(not between the couple). Simply put, a buncha assholes abusing MC, they get their just desserts later on. Also Horror/boddy horror warning as well??? It's not the pretty mermaid kind, it's the borderline lovecraftian type of mermaid.
Ooooohh thank you for the recc♡
This one too. Rad strange but the story's nice
https://www.mangago.zone/read-manga/wadatsumi_no_yometori/
I've been robbed of mermaid seggs...now i gotta look for that one spooky mermaid BL i found mrm...