diff --git a/src/water_map.h b/src/water_map.h
--- a/src/water_map.h
+++ b/src/water_map.h
@@ -68,6 +68,7 @@ static inline WaterTileType GetWaterTile
 /**
  * Get the water class at a tile.
  * @param t Water tile to query.
+ * @pre IsTileType(t, MP_WATER) || IsTileType(t, MP_STATION) || IsTileType(t, MP_INDUSTRY) || IsTileType(t, MP_OBJECT)
  * @return Water class at the tile.
  */
 static inline WaterClass GetWaterClass(TileIndex t)
@@ -80,6 +81,7 @@ static inline WaterClass GetWaterClass(T
  * Set the water class at a tile.
  * @param t  Water tile to change.
  * @param wc New water class.
+ * @pre IsTileType(t, MP_WATER) || IsTileType(t, MP_STATION) || IsTileType(t, MP_INDUSTRY) || IsTileType(t, MP_OBJECT)
  */
 static inline void SetWaterClass(TileIndex t, WaterClass wc)
 {
@@ -88,6 +90,17 @@ static inline void SetWaterClass(TileInd
 }
 
 /**
+ * Tests if the tile was built on water.
+ * @param t the tile to check
+ * @pre IsTileType(t, MP_WATER) || IsTileType(t, MP_STATION) || IsTileType(t, MP_INDUSTRY) || IsTileType(t, MP_OBJECT)
+ * @return true iff on water
+ */
+static inline bool IsTileOnWater(TileIndex t)
+{
+	return (GetWaterClass(t) != WATER_CLASS_INVALID);
+}
+
+/**
  * Is it a plain water tile?
  * @param t Water tile to query.
  * @return \c true if any type of clear water like ocean, river, or canal.