Dark Waters (слэш)

Eren, a young mermaid, meets his dark and intriguing mate, Levi, for the first time by the edge of a large lake in the middle of the wetland forests. Hanji and her team of biologists hide nearby to document the elusive merfolk and their interactions.
Or that one merfolk au where Levi (merman/male/dominant) and Eren (mermaid/arguably still male but in the submissive role here) are a pair of mates living in the wild, meeting for the first time by the lake that Levi inhabits.