This might be info just for me, but .. Just in case. I downloaded the newest version of Paint.NET today, and it seems it has some new features. Specifically, it has this new distort effect called "Dents" and works wonders for roughing up an otherwise clean coastline, creating a bit of extra flavor. You do hafta fidgit with it though. Just thought I would throw this out there.
First, open up or draw a quick coastline image:
Second, open up the Dents Effect dialog, via [ Effects > Distort > Dents ].
When the dialog comes up, you will see something akin to:
Now, you just simply need to tweak the settings, and hit apply.