The Naiads or water nymphs that appear in an episode of Grimm don't have tails the way other pop culture mermaids do, but their appearance is directly inspired by the Hans Christian Andersen story, so they get their own page here.
"More and more she grew to love human beings and wished that she could leave the sea and live among them."
Grimm is a supernatural fantasy drama TV series that premiered in 2011 and its main concept is a police procedural with cases that look a lot like classic fairytales. The Grimm fairytale collection was the obvious jumping-off point, but the show has expanded to include other stories, including "The Little Mermaid".
In the season 3 episode "One Night Stand", two young men, Jake and Dan, are hanging out at a riverside with two ladies they've just met: Sarah and Anna. These two ladies are sisters, and their antics are being spied on by Elly, a third sister, who is deaf. Sarah spies Elly in the bushes and warns her to stay away, but Elly is upset — why did Sarah have to pick Jake, whom Elly is in love with? Sarah dismisses her younger sister, reminding her that Jake doesn't even know that Elly is alive.
The two couples enter the river and things get hot and heavy. Then the two men are dragged underwater, though Jake survives and is dragged to shore by Elly. He gets a groggy glimpse of his rescuer before she flees into the bushes. The heroes of the show then enter the scene, investigating what happened to Jake and his friend, and follow the clues down to Elly, who had stolen Jake's cellphone in order to admire his pictures.
Elly's portion of the story is reminiscent of the Hans Christian Andersen's tale, but the mythology in Grimm happily goes its own way. Naiads are creatures that look for the most part human, but when they're in water their other characteristics are revealed: glowing eyes, gills, and webbed hands and feet. They can breathe underwater and swim unnaturally fast, and they cannot be out of water for more than a day, or else they dry out and die.
As for their society, Naiads are supposed to stay away from humans, but Sarah, Anna and Elly were raised by their single father near people and are considered deviant by more traditional Naiads. Another factor is that Naiad males are sterile, so females need to mate with humans in order to conceive, and the male mates, jealous of this necessity, usually kill the human males once the mating is complete. Hence why Jake and Dan were dragged down by Sarah and Anna's betrotheds. (Side note: I dig how Sarah, Anna and Elly look nothing alike, which fits into this biological necessity of their conception.)