In the name field within the Modify panel type Ground_Cutout. This will be used for making a hole in the outer ground later on as if it can accommodate the Pool Model in its surface.
Re-name the other 3 NGons as Pool Wall, Pool Bottom and Pool Water respectively.
Select the Pool Bottom object and apply an Extrude modifier to it. Set the Extrude parameter as Amount = -4.5 (approximately) in the modify panel.
