Definition of the water level
The depth of the ground water level is defined with an excavation stage, because different excavation stages may have different water levels.
If no real excavation is present, the horizontal ground surface may be defined as an "excavation with depth of 0". Just click the icon "A" for excavation or the menu item Excavation - New and draw a horizontal line from the left or right edge of the footing to the left or right.
Then, in the dialog "Excavation" the water level can be defined (positive depth from z = 0 downwards).