[n00b] how to do shorelines in GIMP?

[teef]

recently got back into mapmaking, using HexKit because it's a nice speedy way to bang a map together without getting up to your eyeballs in techy details.

that said i've found that i want/need to make some tiles of my own to use in the app. that has me wondering how to do shorelines in GIMP (my graphics tool of choice, no-brainer really since i'm on Linux).

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

Coast_127.png

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

shoreline-example.png

i've got the water tiles to use as background, and the land tiles that i have cut my basic coastline from but it's that transition area that i'm really wondering about.
so, GIMP n00b that i am, i'm wondering "how do i do that?"