You need to merge this layer down onto the blurry one. Then go to color>invert to make the black line white. Then Color>Levels and play with the input sliders. Generally input starts at Black 0, Gamma 1.0 and White 255. In RobA's example he has slid the values to Black 174, Gamma 2.29 and White 194 if I recall correctly. For me the main thing to look out for is how much dirt layer you want to show on your coast. You could tweak it by painting the mask later or you can do it by playing with the levels to get the thickness you want. Give it a try and post your results.