Off hand, and please understand that it has been a LOOOOONG time since I worked that tut, but it looks like there was a Land Mask that either wasn't applied, or not applied correctly......I think that because of the "bumpyness" or "balloonyness" of the coasts.

Did you start with your Paint map as a layer in Gimp and then select off of that? If you did you should be able to create a mask that would allow you to keep the coastal layout you had to begin with...I am off to bed, but i will try to find you some better instructions on how to do that later