Attention needs to be given to sky reflection in the water. Just replacing the sky isn't very realistic.
In this rough example, I duplicated my new sky layer, flipped vertically and used Advanced blending mode sliders to get the best blending into the water that I could. Further blending with layer mask touch up, layer blending mode change, opacity reduction and a hue adjustment layer.
Additional changes from original:
Desaturated blue rope, boosted saturation of red paint. Toned down my replacement sky to compliment with the moody foreground.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=26856527
original image by Ray Gerrard: