Type Ctrl (Command on a Mac) + J to copy your selected area automatically and jump it to a new layer of its own. Then go to Edit > Transform > Flip Vertical to turn it upside down. Type V to get the move tool, and, holding down the Shift key so it doesn’t wiggle left or right, drag the top half down until it meets the horizon line. Since in this case it doesn’t quite fit, type Ctrl (Command) + T to transform it, and stretch it until it meets the bottom.
If you don't need to mask off the foreground, head straight to Step 5.