It looks like a not very loaded wide brush painting. That way, you'll see the strokes. For me, it really doesn't fit with the map elements, but that's a matter of taste.

To do this sort of thing with a physical brush is pretty simple: get your base medium ready, load up your brush with dark paint and draw around and across the coastal and ocean areas with long, continuous strokes.

To do this digitally is a bit more complex. Step 0: get a painting tool that can mimic a not very loaded wide brush. Step 1: make a new layer and fill it with a medium tan background color that will blend through the paint strokes as shown above. Step 2: Make a new layer and then, using a dark color, draw around and across the coastal and ocean areas with long, continuous strokes. Step 3: Draw the land areas on layers above the background (if you already have land areas, create the layers from steps 1 and 2 underneath the land layer).