If you find yourself photographing an expansive sky and a large body of water one thing that will come to mind is where to place the horizon. In the middle? Up high? Down low? Horizon placement of course matters in many types of landscape photos. However, in this type of situation it is particularly obvious if you go with a 50/50 split. There's no one right answer. It's more about recognizing your options and deciding what works best for your vision of the scene. But don't ponder this too long if the light is changing quickly...see the second half of this post. No need to make that definitive decision on the spot. Shoot a few variations and then you can mull it over when editing your images.
In the top image I went with the horizon a little below the middle, but I also shot the 50/50 split, see below.
Between these two I prefer the top-heavy composition. I like the greater visual weight of having more sky. Also, with the reflection being darker than the sky, I didn't care for having a large dark tone across the bottom edge. Which one do you like better?
The second set of images focuses on variations of light. The warm early morning light illuminating the marsh grass was fleeting. Less than a minute after I took the shot the light was gone and the grasses were back to being a dark blob in the shade. The sunlight certainly makes the photo. Moments like that are good reminders of how important light is to photography. Also, look closely and you can spot the moon at the top center of the photo.