I previously suggested Secret characters (similar to secret seeds), where your players would gain different abilities based on name, and a mermaid that breathes water was one of them. In my opinion, that would be the best way to implement a player like this, since as mentioned, we would need much more ocean content to make this worthy of being its own seed, which I also suggested in another post called deeper/extended oceans you may like. Still, I personally would prefer if it was an update rather than just a secret seed.
So as for the mermaid suggestion, support but a seed may not be the best way to go about it.