The range limit of DGPS is usually given as 200 nautical miles. But that is seldom achieved. Both the DGPS station and the user have to be in direct line of sight of the same GPS-satellite, topographic and metereological conditions play a role.
DGPS operates on long wave frequencies.

Look here for more. The USCG has a network of these stations.
http://www.navcen.uscg.gov/dgps/default.htm
http://www.navcen.uscg.gov/dgps/coverage/CurrentCoverage.htm