Sew a mermaid tail!
There are several great tutorials online for making mermaid tails.Sasha's step by step video on youtube
is very helpful.
Instead of using fabric from a fabric store, we used a dress from the thrift store. Since you won't need to put your real legs inside, you won't need stretchy fabric and you can make the tail slightly smaller than you would if you were making it to wear.
We turned the dress inside out and drew the tail shape. We then pinned it , cut it out and with the sewing machine, sewed along the lines. This is a very easy sewing project. Even my friend who had never used a sewing machine before managed to sew the tail with only a little guidance!
With the tail right side out, we sewed the fin closed at the bottom and left the top open in order to stuff it with batting.
To finish off the mermaid you'll just need a sea shell bra and a long wig. It's important to have a lot of hair as this will help cover the backpack straps that hold up the pirate.