Now invert the layer (Color->Invert) and blur the image (Filters->Bur->Gaussian Blur) The amount of blur will determine how random the final coast will be. The larger the blur, the more random the coastline will be (in the blurry area). If you have a well defined coast already (like here) use a smaller blur (50px). If you have a poorly defined coast (as is the case when using a random coast technique) make it a large blur.
I blurred my image 25px.

I went back and blurred a few areas more by hand on the main coast to get more randomness there but keeping the detail in the island ring. Note the histogram. What you want is a nice level image, with dark and light more evenly spread out, as opposed to a U shape.
Well, I'm looking at mine and it's completely flat... No idea what the histogram is or why this is important. I guess I'll have to soldier onward...

Okay, I clicked on one of the buttons to see if that helped, and I think it did. By default, mine histogram was set to Linear and I guess it needs to be on Logarithmic. Here's a screen shot: http://i852.photobucket.com/albums/a.../histogram.jpg

So, it's 'U' shaped and the tut says you want it flat, but I don't know how to level it out...

Well, I'll just have to continue with a 'U'-shaped histogram.