Nice work for your first efforts there mate.
Using layer masking you can combine anything really, it's a matter of having the software and the skills to make the most of what you're trying to achieve. Of course the main thing to remember is to ensure the final image remains natural looking and in this case the masking should be releatively easy due to the difference between the water and the grass/trees.
What you might like to consider though is zooming in on some details like the reeds in the foreground with the water, creating a mono image and exploiting the unreal look of motion in the reeds, with comparitively calm water. You could create something really special, with colour, washed out colours or high key B&W. You can add textures too and each of these versions could be good in their own right, or framed as a group showing different interpretations of the one scene. With good images as your start point, there's a lot of options for creation in post, and the beauty of it all is it's your image and you get to create what you want.
A quick question, did you block the optical viewfinder during the long exposures?
Tas