Taylor Swift who miraculously won best album of the year looked absolutely stunning in her mermaid costume. For your own mermaid costume, (if you can't afford to have a dress especially designed for you), I suggest something along the lines of what you see above.
If this is not your style, and you are more into a traditional mermaid costume (ie. more skin showing), try something along the lines of Darryl Hannah's mermaid outfit from way back in the day. Just remember that it is difficult to walk with full fins.
I do not recommend going, however, going the route of Cher in "Mermaids" as the tackiness factor would defintely outweigh the charm of the traditional mermaid costume.