Index: trunk/resources/data/validator/geometry.mapcss
===================================================================
--- trunk/resources/data/validator/geometry.mapcss	(revision 16131)
+++ trunk/resources/data/validator/geometry.mapcss	(revision 16145)
@@ -181,4 +181,10 @@
 area:closed:areaStyle[landuse=reservoir]                        ⧉ area:closed:areaStyle.water_area {
   throwWarning: tr("Overlapping Water Areas");
+}
+
+/* Water area inside water area (spatial test) */
+area:closed:areaStyle[natural =~ /^(water|wetland|coastline)$/] ∈  area:closed:areaStyle.water_area,
+area:closed:areaStyle[landuse=reservoir]                        ∈  area:closed:areaStyle.water_area {
+  throwWarning: tr("Water area inside water area");
 }