The coastline could be a bit more wrinkly, but given the hand-drafted look of this map, the smoother coastline isn't bad--the cartographer probably doesn't have super-accurate measurements, and at this scale only a few inlets would probably be recorded. If you look at maps from the 17th and 18th centuries, you see smoother coasts like that one.

The darker-colored area in the middle almost looks like a stain on the parchment rather than a region. Maybe a pattern, or a border rather than a dark spot would be better.

For the desert, you could try dunes, but I think that might start to overwhelm the information. Instead, try a speckled pattern--just some dots scattered across the desert area and a label.