@ TheGuitarist: Actually I tried making something pretty similar to that scene a while back LINK
What I did for the water basically was first just draw a lot of lines with different shades of gray over a black background using a small brush size. Then I used fragment blur which gave me the basic look that (with quite a bit more experimenting) can give a pretty decent ocean-look. In the image below i then blurred and scaled the "water" and added a simple little moon.
@ Skitzo: I like that planet, cool idea using different colors like that. For learning spaceart techniques I think I would search for some photoshop tutorials. There's really a lot of material out there and I'm sure a lot of the techniques are easily transferred to Paint.NET. I'm no expert on the subject though.
@ Aislin: I like both of them, especially the first one. It's a nice crop and the simple background fits the subject very good. In the other one I thought I would like to see a bit more of the persons face, but seeing the original image I realized the hands would come in the way. It looks good though, maybe just a bit dark, but good job anyway.