If it’s meant to be more of a costume, there are better ones out there.
If it’s meant to work like existing swim flippers, but mermaid-style, it seems dangerous. Having your feet bound while underwater seems like a bad idea. Maybe if it was easy to kick it off if you need to. …but if it was easy to kick off, it would probably just fall off when you don’t want it to.
If it’s meant to be more of a costume, there are better ones out there.
If it’s meant to work like existing swim flippers, but mermaid-style, it seems dangerous. Having your feet bound while underwater seems like a bad idea. Maybe if it was easy to kick it off if you need to. …but if it was easy to kick off, it would probably just fall off when you don’t want it to.
these are called monofins, and are used in competition free-diving.
http://en.wikipedia.org/wiki/Monofin