I like that tree.

I think you should try the girl staring into the water idea, but even if you don't, I think you could still get the sky and water in with what you have already. I adjust your palette a little, and changed the dirt bank a little to make it look a little more dull.

I'm no pro, but I have picked up a few things. First off, your color ramps aren't connected to each other at all. A general rule is to make your palette connect to have a unified feel in the overall image. You can start with different ramps and connect them after, or start with one color and build your ramps out from that, but you should generally have all of your colors connected somehow, as in the below example.

In pixel art, using less colors is mostly about finding what colors are similar and how you can merge them and adjust the ramps they are in. As an example, I merged the darkest blue and darkest green into a blue-green that can connect those two ramps. This removed one color, which I replaced with a "black". Generally, you don't want to use pure black or white, because they are powerful colors. Instead I used a dark purple to act as a black, and also to connect the blue and orange ramp. I didn't change your white, but the same applies to it, so you might want to change it for practice.

Lastly, with limited colors, some things will have to use colors that might seem off at first, but can be made to fit, and almost all if not every color should have multiple purposes. For example, you could use the color ramps below to make the skin, dress, water, and sky, and use the black as an outline to make things of the same color stand out in front of each other. Choosing the right colors depends on what you want to use those colors for, so you might need to adjust your palette as you get closer to finished.

This still leaves you 3 colors left, but if you used the black and dark blue-green as hair, and the white for the old hair, you could do everything you wanted with the thirteen colors here. If you do add the other three colors, try to use one per object. Such as, 1 color for hair, 1 for skin, and 1 for the water, and use your existing colors to make ramps with those. Also, remember to make it connect to your other colors.

If you want to read more on palettes, here are a few tutorials on them I've liked.

Nice, short video, and a good channel to boot.

https://www.youtube.com/watch?v=QhgSM_tnPM4

This one I really like, and suggest going through all the chapters.

https://opengameart.org/content/chapter-5-color-palettes

This one is pretty long, but good.

http://www.slynyrd.com/blog/2018/1/10/pixelblog-1-color-palettes

And this is a link to a bunch of other links to palette tutorials.

https://lospec.com/pixel-art-tutorials/tags/palette

Good pixeling to you, and good luck in the contest.

 Keep the sky a simple flat blue, and do similar for the water, having a flat dark blue, with a light blue showing the shoreline, maybe some wave shapes if you get time. For times sake, the two girls facing each other would probably be faster.