Now for the fun—making the water more realistic. Click on the layer thumbnail for the upside-down sky. Then go to Filter > Blur > Motion Blur. Choose an angle that’s nearly horizontal (4 degrees or so) and a distance you like. Click OK. Now run that filter again, this time choosing a more vertical angle, such as 60 degrees. Finally, darken the water. Make a Curves Adjustment Layer, and lock it to the water layer by Alt (Option) + clicking on the line between the Curves and the water layer. Bring down midtones until your water looks real.
If you hate making selections, be sure to start with an image with a simple foreground, or just pretend the whole bottom half of your photo was the lake.