Can't be more like the GPS in a car... the GPS in a car knows that roads are the only "routes" available... and except for mapping errors, would never try to route you through a building or across a river where there wasn't a bridge.

So why is it that marine systems don't seem to know where the "hard crusty parts" exist? Seems to me a GIS person could easily map an outline of all the land masses and put them into a database.