Legend of Heaven and Earth: The Mermaid Beauty

Strictly speaking, this twenty-episode Taiwanese show doesn't have a mermaid, but it still sort of counts. Based on the story "Guanyin Yulan Ji", the main character is Xiao Lin (Yuki Hsu), a carp spirit who lives in the household pond of the wealthy family of the beautiful and gentle Mau Dan. Xiao Lin has been practicing cultivation with a teacher to elevate herself, gaining skills in martial arts and spiritual energy that enables her to gain a human-like form. Xiao Lin is also a playful spirit, so she uses her abilities to get up to all sorts of tricks.

Xiao Lin loves Mau Dan, so she secretly helps her pursue Zhang Ziyou, a poor but kind scholar who has won Mau Dan's heart, though Ziyou's low social standing has Mau Dan's parents rejecting the match for another more prestigious one. Xiao Lin tries her best to help the couple through their trials, but this is cut short when Mau Dan dies tragically. Xiao Lin, wanting to help Ziyou, shapeshifts into Mau Dan's form, but the deception gets out of control and Xiao Lin ends up falling in love with Ziyou herself.

It's a love triangle of sorts, but one where Xiao Lin's non-humanness informs her choices despite her good intentions, and she learns some lessons along the way. Ziyou eventually realizes that Xiao Lin, although not a human, does have a good heart, and he faces a trial of his own when Xiao Lin ends up becoming a human herself with amnesia.

All the spirits in the show have aspects of their animals selves while in their human shape, and Xiao Lin has a golden mermaid-like tail as befitting a carp, and there are a few nice little sequences of her swimming in water. I've embedded one of the episodes below with a timestamp to one of these sequences.

The whole series is on youtube, but without subtitles:  Legend of Heaven & Earth: The Mermaid Beauty playlist