so I just found out one of my free diving friends worked on the free diving show, being the safety free diver for the stunt actress and the main actress. She said the mermaid tail is a costume, not CGI and that it took a lot of work to get the kick right because of how it's made! I asked her specifically, is it a costume, CGI, or a mix of both? And she said costume and she posted some BTS pics but I can't post them there. IT'S REALLY ELABORATE. I can't confirm, but It looks like the same studio that did the h20 tails I'm going to look into it.
This was a TV series called "The Deep" but it's been changed to Siren. so that's why IMDB will change