Mermaid costumes tend to be form-fitting, partly because the bottom part has to resemble a tail as much as possible. Sometimes the costume comes as a one-piece dress, other times, the top part is either a corset or a bikini top, and the bottom is a fish-tailed skirt. Some of these costumes are very tight around the legs, so if you plan on doing a fair amount of walking, you should make sure you can move around comfortably while wearing one.
Here’s one that caught my eye. I love how shiny this version of the costume is:
Sexy mermaid Halloween costume pictured: Under The Sea Sexy Mermaid Costume by Forplay