I like the scene and the choice of color tone works well. Seems just a tad washed out, like it could use a little more contrast. ??? Just a thought. Good luck in the challenge.
This evokes a real sense of calm, and I think the sepia works well. Perhaps a little burning on the clouds for a little more impact? I also don't know why you didn't use the maximum size of 800 pixels.
The toning works well for this in my opinion. Make sure you use the full size available - this can be 800 pixels in width - much more effective that way.