I was just going to add, if that's a bit too rough, you could use the fuzzy selection tool again and play around with its settings. In this step I selected the empty area, inverted the selection, and then filled it again.
Screen Shot 2015-04-16 at 11.21.12 AM.png
For sure ask me if you have any questions and, like I say, someone who knows their GIMP may well come along with a simpler/better method.

cheers,
Meshon

Edit: I should mention, this is not going to give you realistic coastlines, but it will break up the smooth and give you a bit more to work with.