Cool drawings! You seem to have a pretty good grasp of rendering texture and such. I would work on structure though, if you build something up first by making 3-dimensional shapes to make sure it makes sense, it ends up coming together a whole lot better. The mermaid for example looks really nice, but if you took away her hair that tail would be bent at a really strange angle. Just something to consider.
Still, good stuff. Keep it up!
P.S. Just to say, "light effects" are anything but realistic. Drawing from life really helps you understand how light and shape work together. If you get a sketchbook and just start drawing random things you see (in real life) when you're bored, your understanding of objects will increase tenfold.
