Mapping Toolbox has two functions for plotting filled polygons: geoshow and mapshow, as well as a low-resolution coastline data set, coast.mat. Use geoshow when working with a projected map axes. For example, if you load coast like this: coast = load('coast.mat'), then you could call geoshow like this: geoshow(coast.lat, coast.long, 'DisplayType','polygon','FaceColor','green').
You mentioned contourf (rather than contourfm), so you're probably in a ordinary axes. In that case, use mapshow instead, and swap the order of the first two inputs. For example: mapshow(coast.long, coast.lat, 'DisplayType','polygon','FaceColor','green').
As Mark mentioned in his answer, one way to work in higher resolution is to import GSHHS coastline data via the gshhs function.
