Okay, I couldn't help myself.
So I first addressed the composition.
Consider cropping in and focusing on what you want the viewer to be looking at.
There is a lot of sketchy brush work here but that's okay cause you said it was a sketch.
Also if there is a ritual in the water of some kind and that's the story you want to tell: in the image provided with the magenta line (eye path) in it, in the number 2 circle, you could put anything. A clawed skeletal hand, sword, a fish transforming, whatever.
So it would go, 1, 2, for focal points, and the 3, is just shapes of the dress and the tree leading the eye back to the main focal point over and over again.
I did not over render it. I only used the tone swatches that I left there next to her. An overall shadow, a darker occlusion shadow, and the light source. Those hue variation brushes you used are fun but can muddle your values really quickly and distract you with cool mark making.
It seemed like the hair was hiding a lot of the body. You don't have to stick with this design I made, however remember it is a good opportunity to frame faces with hair, plus why hide that beautiful anatomy opportunity.
I painted the rest of her body shape with two tones,
then the dress over it on another layer.
The detail on the dress can be the lighter white color you use there. Using two to three tones (when I say that I mean values, but they have color here of course) is all I did to set up the painting, and color grabbing with the brush tool.
Remember take all critiques with a grain of salt as you are the artist on this, not me. And this has my aesthetics now.
Hope that makes sense and helps if you have any questions let me know.
Cheers
