This post it's just to explain briefly how I made my mermaid tail and how I managed to make it good for swiming.
First thing: get the fabric. In my case I chose a velvety-like sequined turquoise fabric . Make sure you buy the right amount of fabric to cover your legs from hips to toes (and like 40 cm. more to make the fin). It's important that the fabric is elastic, if not, it will be very difficult for you to get into the tail or to move in it, because it's supposed to be very tight.
Then you just draw a pattern with your leg's silhouette and add the thin's shape (I used a scuba fins, If you want to swim with the tail better get ones too). The idea is to buy a mono-fin, you know, like the normal ones but just as if they were sticked...just as a mermaid. The thing with them is that they're a little bit expensive...so, I bought the normal ones and joined them with tape hahaha.
When you have the silhouette drawn just mark it in the fabric, cut it (always add some cm. for the seams), and sew it with the fins in it. That's supposed to work pretty easy. Then you can put it on and look for if it needs some alterations.
Other advices: add an elastic ribbon in your hips if you want the tail not to fall. AND, make sure there's some cm. more than your actual pattern in paper shows because if not, you won't get in the tail. I marked the shape to tight and I had to add lots of parts (at least it looked cool, but it was a lot of extra work).
Optional: make a 'rear' fin with more silky-like fabric, just to adorn (it looked cool in the water!).
Aaaaand, that's the result!
Yeah, that's my living room xD That was for you to seet it better.
And here you can see the rear fin I added.
I hope this was useful to you ^^