note this is *salt water*
see also http://wiki.openstreetmap.org/wiki/Proposed_features/shelf
HB: tagging as natural=coastline, which is appropriate for Chatham Island. Will have to review for mainland. (use natural=water if small and well separated from the seaward coastline) Is Sutherland Sound in Fiordland included? [no]
for the mainland it seems to be more inland waterways.
linz layer descr: "A sheet of salt water separated from the open sea by sand". So not coastline, but natural=water + saline=yes.
RP: if it's separate from the sea, i think it should be natural=water. see the wikipedia page for more info
HB: at least for Chatham Isl. it's connected to the sea via a small cut:
Uploaded for Chatham Island 29 April 2010 (HB); had to reverse all ways by hand.