Here's the NEWSDAY Community Profile:
Buying in Water Mill -- Newsday.com (http://www.newsday.com/classified/realestate/local/ny-rebuyn25567294feb08,0,4266900.story - broken link)
Concomitant with the April 2000 Federal Census, Watermill had its name changed to Water Mill, Water Mill is a hamlet (an unincorporated area) in the east part of the Town of Southampton, in the southeast part of Suffolk County on the South Fork.
Beginning on the north and moving in a clockwise direction, the Hamlet of Water Mill in the Town of Southampton is bordered on the north by the Hamlet of North Sea and the Hamlet of Noyack; on the east by the Hamlet of Bridgehampton and Mecox Bay; on the south by the Atlantic Ocean; and, on the west by the Village of Southampton and the Hamlet of North Sea.
The Hamlet of Water Mill has a different border than does the "Watermill, NY 11976" postal zone (i.e., a place can have a "Watermill, NY 11976" mailing address and not be in the Hamlet of Water Mill and a place can have other than a "Watermill, NY 11976" mailing address and be in the Hamlet of Water Mill): places that have a "Watermill, NY 11976" mailing address that are not in the Hamlet of Water Mill are in the Hamlet of Noyack and the Hamlet of Bridgehampton; and, at the same time, there are places in the Hamlet of Water Mill with a "Bridgehampton, NY 11932" mailing address.
For a good set of town-by-town maps showing all the villages and hamlets in each of LI's 13 towns (3 in Nassau County and 10 in Suffolk County):
//www.city-data.com/forum/long-...-resource.html