My process for making a map in Gimp always results in a coastline saved as a channel. To make coastal rings, make a selection from the channel, grow it a few pixels, stroke it. Grow it and stroke it until you have as many as you like. You can make each 'grow' a pixel or two larger than the previous to get wider spacing as you move further out to sea.

Myself, I'm always at sea, but that's another story...