Mermaids, Wolves and Vampires - H2O Just Add Water/TVD crossover (2018)

Emma, Cleo and Rikki are three mermaid friends living in Australia. After defeating their enemy, Charlotte, they think that everything will go back to normal. But Cleo isn't done with Charlotte yet, and when she sees her talking to Lewis, she wants to get revenge. She accidentally kills her and then strange things start happening to her. There is a brand new reason why the girls should fear the upcoming full moon...

This is taking place right after the ending of the last episode of season 2, when the girls ran off to the sea.

I have made two important changes to make this work.
1) Cleo/Hayley wasn't a wolf yet when she became a mermaid, but she did always have the werewolf gene.
2) Emma/Rebekah could gain magical powers despite being a vampire because she got it from the full moon, which isn't bound to the supernatural laws tied to the Earth.