The use of the filter would allow the sky to be a slightly darker, to match the richness of the reflection. If you look at the last three reflections, you'll notice that the reflection is slightly darker that the source. Water acts as a natural filter for some of the light. If you want a balanced image, the graduated neutral density filter is a great tool.
Now, working with the same principal of slowing down my shutter speed in order to soften the water, with this shot of Louise Creek I went even further. The shutter speed here was 30 seconds long
1. Have your tripod on rock-solid ground. 2. If it's windy, lower the tripod (shorten the legs) if possible. Mke it as stable as possible. Unless absolutely necessary, never extend the neck of your tripod up when shooting in low light or with a slow shutter speed.