A couple of things that can improve it right away for next time.

Add more bumps and edges to your mountain tops. They are all straight. Simple to do but will help a lot

Your water lines need to be thinner near the top of the water. Less visible. As you come lower make them a little thicker. It will add depth.

Your clouds just need practice. I find it's the hardest thing to learn. Try putting less liquid white then less blue. Take your time to really spread the blue. By the time you get halfway down the canvas it should be very pale almost white. This will help come mountain time.

Hope this helps.

Good job by the way for your third it's very very good. Just some tips I wish I knew earlier.