This is a very good shot, Falstaff. I also have the 10-stop B+W ND filter and I know that sometimes it's hard to achieve natural colors. I would play with tint / WB sliders a little, I have a feeling that the water is a bit greenish...

Did you use ISO 50 to achieve a slower shutter speed? On most cameras shooting below ISO 100 decreases the dynamic range. Some shadow areas seem black.

Anyway, congrats. I hope to visit Iceland one day too.