[n00b] how would you do this shoreline?

[jhoo]

getting back into mapmaking, using an app called HexKit. normally it's a nice speedy way to bang a map together because the tiles come pre-made and you're spared a bunch of techy details.

that said i've found that i want/need to make some tiles of my own for a bit of geography specific to my own world.
that has me wondering how i would do shorelines in GIMP.

here's an example from an excellent tile pack called HexMarch:



if you zoom in on that you can see the way he's done shorelines:



i've got water tiles to use as background, and the land areas that i have cut my basic coastline from but it's that transition area that i'm really wondering about, that land-to-water area that i think looks quite good.

so, GIMP n00b that i am, i'm wondering "how do i do that?"