Via NASA's Expedition 26 crew:
While in orbit aboard the International Space Station, an astronaut from NASA's Expedition 26 crew snapped this clear shot of the tidal flats and channels near Sandy Cay, on the western side of Long Island and along the eastern margin of the Great Bahama Bank. Composed mainly of carbonate sediments ringed by reefs, the sediments were formed mostly from the skeletal remains of organisms settling to the sea floor; over geologic time, these sediments consolidated to form carbonate sedimentary rocks such as limestone.
The continuously exposed parts of the island are brown, a result of soil formation and vegetation growth. Light blue-green regions indicate shallow water on the tidal flat, while the channels and areas to the south of the island have a vivid blue color that indicates deeper water.