Next we need our sky. Open the photo you're using as a sky donor, and using the rectangular marquee selection tool, select a large chunk of sky. Press Ctrl + C to copy this to the clipboard.
Going back the the picture we were working on, press Ctrl + V to paste a copy of the selected sky into the image. If the photos are of different sizes, you may need to alter the size of the layer to fit. In Photoshop Elements this is simply a matter of selecting the move tool and dragging the corners to match the other picture, but it may be more complicated in some other programs.
Next, create an exact copy of this new sky layer. Either use the Layer menu, or simply right-click it in the layer palette and select Duplicate Layer.
The duplicated sky layer is going to become a reflection, so it will need to be flipped vertically. You can do this via the Image > Rotate menu.
Now comes the only tricky part, positioning the layers so that they look realistic. First, we need to get them in the right order. In the layer palette, drag the two foreground layers so that they are positioned above the sky layers. Next, use the Move tool, select the flipped reflection layer and drag it downwards so that it fills in all the semi-transparent areas in the reflective water. You should still be able to see the detail on the foreground layer, as well as some details in the reflection. Finally, position the new sky so that it looks natural and realistic.
You can touch up any areas of missing reflection by using a small soft eraser brush on the middle foreground layer. Once you're happy with the result, flatten, save, print and sent to a jealous relative.