Well now, that's pretty easy... Pilgrim [/John Wayne]
I always start out with a black and white base map, where the land is white, the sea is black. I make a selection of the land on a new layer and blur it very slightly, so that the coastal rings/lines aren't too sharp and precise. I expand that selection by two pixels, fill it with white, set the fill of the layer to 0%, then add a 1-pixel interior stroke. The other two lines are made by making two more layers, expanding the selection to 4 pixels on the second layer, 6 pixels on the third layer.
Then I make the innermost ring/line layer's opacity 75%, 50% on the second layer, 25% on the third layer. Then merge all three layers and you're done. Sometimes I'll lower the opacity of this merged layer a bit if it seems too dark.
Edit: The black coastline sits on it's own layer; you may have to alter the depth of the initial selection to reflect how close you want the innermost line to be to the coast.