A quick way to make beaches with layer effect (I did this in a 600x600 image as an example- it can look much better when done with a larger image).
This works with Photoshop, and *should* work with gimp too, although names may have been changed to protect innocent layer effects.
I usually prefer to draw out most things- like foamy waves along the shore- but this works in a pinch, or if you don't have the tablet/time.
Part 1 -> Part 2