I've seen several mermaids use belly dancing scarves around their waist so those seem to be popular.

I would also cruise sites like Amazon and look for really cheap fabric scarves you could use. You could even check thrift stores and get an old belt or scarf just to try it out or to up-cycle it.

I made myself a latex belt and when I went to mermaid in it, it wouldn't stay put around my waist so whatever you wear around your waist needs to be tightened down really well and possibly have several areas where it ties or hooks to keep it from shifting.