OSM Page: http://wiki.openstreetmap.org/wiki/Tag:landuse%3Dreservoir
NOTE: these are covered or uncovered in OSM, but no differentiating tag.
see also tank_pnt and tank_poly.
"landuse" isn't great, but man_made=covered_reservoir is perhaps too much of a singleton.
uploaded for the mainland 24 Feb 2011 in changesets 7379241 and 7379272 (HB).
HB 22 Aug 2012: updated to match new linz data release field names.