I've finally completed my mermaid concept, though I admit I might continue to tinker with it. After dropping it into greyscale to take a look at the contrast I had decided that I needed to make it murky and that I didn't like the spear.
So I looked online at different types of fish spears and after finding one I liked, looked at the possibility at making it appear a bit more organic. I didn't want to use wood and metal, since it seemed an unnatural element for a sea-creature to use (though the idea of pillaging materials off a ship wreck was pretty intriguing).
In the end I wound up using the skull of a marlin as the spear, adding a bit of a spike from a narwhal to the center of the 'spear head'. I'm pretty happy with the way this turned out overall. Now to submit it into the challenge!
Don't forget to vote!