Slippery as an Eel

Of all the sea creatures in the world, alongside dolphins, octopuses, sea otters, and whales, the sneakiest sea creature alongside the shark would be the moray eel. In nearly every form of media, eels are depicted as being sneaky creatures that have a propensity for tricking other sea creatures. It's a sure sign that they cannot be trusted.

Not to be confused with: Psycho Electric Eel

Examples of Slippery as an Eel include:

Anime and Manga

In One Piece there's Hammond, a villainous Small Name, Big Ego character who's also a Daggertooth Pike Conger fishman.

Film

Flotsam and Jetsam from The Little Mermaid are a prime example of this trope.
There's an eel in The Water Babies film who's the lackey of the evil shark king. Don't recall if he was particularly sneaky in the trickster sense, but he was certainly a sycophantic brown-noser.
According to the song "You're a Mean One, Mr. Grinch" from How the Grinch Stole Christmas, the Grinch is actually "cuddly as a cactus and charming as an eel."
"You eel in snake's clothing!"

Video Games

In Freddi Fish, Eddie the Eel is depicted as a trickster who prevents Freddi and Luther from trying to go where they needed to, unless he is bribed.