Hey Thijs,
I like what you did here comp wise....balancing the rocks and using the long SS to make the water silky. The image appears a bit flat overall so I wonder if adding a bit of contrast or tweaking the water and sky seperately will bring this to its full potential. Just a few thoughts to ponder but let's see what others chime in.
I am good with just a graduation of color in the sky for these types of images. I agree with Roman that contrast and tonal separation needs additional work.
Hi Thijs, Good suggestions by Roman and agree on liking the way the comp is framed but as stated the image needs more depth. The soft water flow due to the long SS is appealing along with a low shooting angle...