I came across this article which I thought I'd share, so if you haven't your ND filter handy for that beautiful sunset water shot or for any other occasion you're needing one, just use your phone with your camera and you can take a much better photo than what you would without the filter.
Here's the link to show you how:
http://www.picturecorrect.com/tips/how-to-use-your-smartphone-as-a-graduated-neutral-density-filter/
It just goes to show how versatile your phone can be!
Cheers!