The mermaid has a special outfit when she is wearing her tail. I can only access the special outfit in Stylist.
She got the sunglasses originally because she was wearing them with her swimwear outfit. But removing them from swimwear doesn't remove them also from her tail. So it's a bit of a problem. Best not to turn a sim into a mermaid if they are wearing sunglasses for swimwear. Only way to get rid of them is send her swimming and force her into stylist while she has her tail.
It is best to have the dive lot made specially for the lot where you want to place it. This is because of varying ocean depths. I could put it onto the Exchange but I think a lot of people who downloaded it would have big problems as it really only will work properly on the lot I made it for.
When I get this tutorial video working you'll see why. I think it's a lot easier to just make one than to try to place one that wasn't made for the specific lot.