Photo courtesy of http://3-fins.com/
The most popular brand is called 3-Fins, and they were created by a woman in Vancouver, British Columbia named Monika Naumann. She started making them after she noticed how obsessed her daughter was with mermaids.
The mermaid tails replace regular bikini bottoms . . . you still have to wear a top. They sell for $245 but, apparently, they're not just for lying around and having people confusedly stare at you . . . you can actually swim in them too.