In the Top viewport, click and drag from the top-left corner to the bottom-right corner to create a plane.
In the Front viewport, move the plane a little to the down. It can be placed a little above or below the Water. In other words, water can be placed a little below or above to the ground level depending on the desired water level (height) of pool.
Now using the Select by Name option, select Ground_Cutout object . It is important that you use Select by Name option to ensure that the correct object is selected. Otherwise, selecting a specific object in viewport directly may be difficult.
