The other way of getting perfectly smooth water is to use a long shutter (say 5 to 30 seconds), which will blur anything moving, including any little waves. But, you need a rock-solid tripod, and all the usual vibration-reducing techniques (mirror lockup, remote release, etc.). From a composition point-of-view, I would aim the camera slightly more down, so that the horizon line is at the upper 2/3 line (ie, less sky, more water), because the light reflections in the water will probably be more interesting than the featureless sky (unless you can arrange for some dramatic clouds!).
Edit: didn't notice that you used 15 sec. So you already know about that method of smoothing the waters...