Add new sky as a new layer.
Select the sky from the original image using the magic wand. Use the selection to create a mask for the new sky layer.
Duplicate the new sky layer and flip vertically. Use this layer to add detail to the water in the original image.

forum discussion http://forums.dpreview.com/forums/read.asp?forum=1006&message=38837887&changemode=1

original image by ISeek: