diff --git a/specs/udmf_zdoom.txt b/specs/udmf_zdoom.txt
index bdd70322e..4fc57f856 100644
--- a/specs/udmf_zdoom.txt
+++ b/specs/udmf_zdoom.txt
@@ -186,6 +186,7 @@ Note: All <bool> fields default to false unless mentioned otherwise.
       soundsequence = <string>;       // The sound sequence to play when this sector moves. Placing a
                                       // sound sequence thing in the sector will override this property.
       hidden = <bool>;                // if true this sector will not be drawn on the textured automap.
+      waterzone = <bool>;             // Sector is under water and swimmable
       
       * Note about dropactors
 
@@ -328,6 +329,9 @@ Added back locknumber property.
 1.20 25.02.2012
 Added arg0str thing property.
 
+1.21 09.08.2013
+Added waterzone sector property.
+
 ===============================================================================
 EOF
 ===============================================================================
diff --git a/src/namedef.h b/src/namedef.h
index 44fe35e79..f2d601eb5 100644
--- a/src/namedef.h
+++ b/src/namedef.h
@@ -442,6 +442,7 @@ xx(Alphafloor)
 xx(Alphaceiling)
 xx(Renderstylefloor)
 xx(Renderstyleceiling)
+xx(Waterzone)
 
 xx(offsetx_top)
 xx(offsety_top)
diff --git a/src/p_udmf.cpp b/src/p_udmf.cpp
index c1d18fd4c..91e4c8565 100644
--- a/src/p_udmf.cpp
+++ b/src/p_udmf.cpp
@@ -1326,7 +1326,11 @@ public:
 					continue;
 
 				case NAME_hidden:
-					sec->MoreFlags |= SECF_HIDDEN;
+					Flag(sec->MoreFlags, SECF_HIDDEN, key);
+					break;
+
+				case NAME_Waterzone:
+					Flag(sec->MoreFlags, SECF_UNDERWATER, key);
 					break;
 
 				default: