My first tutorial and my first gimp map!

I've scanned my hand draw map to use as coastlines then I have followed the tutorial, creating few dynamic brushes.

Here's my unfinished map so far. Still need to add water (rivers, lakes), cities and colors.

The problem I have is that I can't figure out how to separate land mass and sea. The land mass isn't closed so I can't select it with the select tools, there're too many cuts in the coastlines. Is it an easy way to do this? I don't want to retrace all the coastlines with a pen tool, my mouse isn't as accurate as my right hand.

Thanks for the great tutorial.

World_Arnd_2.jpg