Manga Review: By the Sword volume 2 by Sanami Matoh (2005)

The plot of the series follows a half-demon bounty hunter called Asagi on his journey to find the wife of the demon trapped in his magic sword. Along the way he meets various creatures who either help or hinder him. The first encounter entitled simply "Mermaid" has our hero encounter Mizuki a mermaid come to land to track down her sea-dragon father's demon orb which has been stolen. She takes a shine to our handsome hero and offers him her body as well as various sea treasures if he'll help her. He refuses anything more than the treasure, being attached to his companion Kaede (daughter of the demon in his sword).
Only two full tail images, but they are gorgeous. I like Sanami Matoh's artwork anyway, but it's so nice to have her draw a mermaid.
Mizuki has legs on land and the fluted ears common to most Japanese manga mermaids.
For those interested, you can read both volumes of the series online here