Actually, it's a combination of gimp and inkscape. Essentially, I use gimp to make the coastline, put down an airbrushed layer of black along the coastline in a lower layer, then fill in below that with the ocean pattern. Then I save it as a .png and import it into Inkscape. Once that's done I do a bitmap trace and then simplify the whole thing. That's pretty much it.

Also, I decided to re-jigger the map a bit because I needed to use thinner line strokes for the land features, which looked really wonky with the thick lines used for the coast and sea.