If I had mermaid hair, this would be my go-to style. I cant wait to grow my hair long enough for the "fishtail". It simple, yet fun and elegant. I love to fishtail peoples hair and envy the result. My biggest tip if you want the tight look, would be to make sure you pull tight all the along the way. Also, a tip for the end is once you have secured the hair tie, grab a little piece of hair full of a couple strands from the middle of the un braided tail with one hand, then with the other hand, grab the hair tie and push up. The small strand should stay and the hair "scrunch", this leaves you with the messy and fat fishtail with a more couture look (seen on the model in the first picture). Pull the tie back down a bit and style as you wish as far as thickness and length! Here are some fishtails I love...
Here is a step by step chart, showing how to fishtail!