Content warning: There are some gruesome/violent things in this summary.
There are plenty of fantasy realism creatures in Lost Girl, an on-going Canadian drama TV show that premiered in 2010. The main character of the show is Bo, a succubus who is figuring out her role in the world between the dark and light, and along the way she and her friends investigate cases that involve supernatural creatures. A season 4 episode that aired in 2014, "Waves", introduces merfolk to the supernatural mythology of the show.
In the episode's case, a high-powered executive named Diana (Karen Cliche) is swimming when she's attacked by a creature that cuts off her legs. Diana goes to the heroes' detective agency for help, and with Bo involved in another storyline, her friends Kenzi, Dyson and Lauren take on the case. Kenzi and Dyson go undercover into the office building, where they discover a lab with creepy tanks containing legs, and that the boss, Darren (Brandon Firla), used to be a merman. Darren obtained a legs "donor" (who now works as his assistant) and had those legs grafted on to him so he can live among humans. He believes that his sister is out to kill him.
Merfolk in this universe are vicious creatures, make no mistake. Dyson is aware of the dangers but pop-culture maestro Kenzi is too caught up in the idea of mermaids as fantastical folk who comb their hair with forks. There's another Disney version shout-out when Kenzi meets Darren, and when he explains how he obtained his legs, Kenzi says wistfully, "You wanted to be where the people are." (Heh.)
Lauren acts as bait in the same swimming pool to lure the mermaid out into the open. The mermaid, Dominique (Kate Todd), is caught, and explains that merfolk are gifted magical legs for one year to explore land, but must return to the sea afterward. Merfolk also cry tears that turn into salt pearls, and once they weep eighteen pearls they're allowed to take a life. But she's not after Darren's life — she only wanted to punish Diana, who as it turns out is her sister and also a former mermaid, as she promised to find Darren but failed to return home as well.
There's a showdown slash family reunion when the mersiblings turn on the detectives who brought them together. But the day is saved by Lauren's quick-thinking — merfolk are mortally allergic to tap water.
There's no underwater scenes in this one, but the mythology is interesting in its dark turn. The merfolk are all quite beautiful (that said, most of the actors that appear in this show are beautiful) and though we don't see Dominique's tail action it's quite muscular and solid instead of wiry.