I'm not RobA, but he's my hero and I use his techniques a ton. This problem could be because the bigger your map gets (i.e. much over 2000x2000), gimp's native clouds aren't detailed enough to give a good coastline (or other random effect). Try downloading and playing around with Felimage Noise (for coasts I usually use a 10-15 size, and turn the lacunarity up and the Hurst exponent down). It's also pretty important to make sure your fuzzy select tool is set to "Select Merged" and a 0 threshold.