I made this costume for a little Barbie video that I made. It was super easy and didn't take too long. I would recommend using a stretchy material and sewing it closed (instead of using velcro) if you plan on wearing it out.
Everything was hand stitched (I don't have a sewing machine) and the materials were very cheap...most from ebay or the fabric store.
Good luck! I love mermaids and it was really fun to be one...and even funnier to see everyone's reactions at the beach ;)