I love mermaids. I like the sims mermaid but the thing that has always bugged me is that the mermaids can't sleep or basically live under water. I have never modded before but I would love to start by making a mermaid household lot. Is there anyone interested on this project(just an idea at this moment)? It would be an ambitious project because someone would need to make working tails for babies, toddler and children. There would also be making under water needs objects plus animations. Honestly if someone wants to take this idea and run with it go ahead.

I know this isn't exactly what you're asking for but there's a mod here on MTS that allows mermaids to sleep underwater (underwater caves or corals) or on the surface of the water. It also allows them to eat fish and such without catching them first and they can play solo or together, filling their fun. Those three things really make a difference, as they can pretty much stay underwater for longer and, if you want them to, they can even live in the dive lots without having to visit the surface too often, being most their needs can be fulfilled there. If you don't already have this mod, please feel free to try it out, so you can get a better idea of what you'd need to do with your project. Unfortunately, I'm no modder but if there's a way I could help, I'd be glad to do so!

EDIT: There are tails for babies (1 , 2) and children but I'm not sure if they're functional. It's definitely a start though.