Brown water: Brown water refers to the highest point of high tide to the end of the littoral zone. Think coastal area. Many times the term brown water navy refers to military/police ships that patrol harbors and rivers.
Green water: Green water refers to the region between the coastal zone (brown water) into the ocean past continental shelf. Green water navies are regional powers who can extend their fleet a limited range. Think South Korea, India, and the United States Coast Guard.
Blue water: Blue water means the open ocean. Blue water navies are those who can travel the world while displaying overwhelming force. Prime examples are the United States Navy, the France, and the People's Republic of China (which will soon be a blue water fleet).