The "rule of thirds" is a tool which, while useful, is not a rule for every shot. It is very good at adding extra dynamism to a composition, makes things look more lively. But, as in your photo, if you are trying to emphasise the opposite, ie stillness and calm, placing the mirror line in the middle does this. If you had chucked a stone in and got ripples, then moving your camera closer to the water and having two 3rds of that would have emphasised the movement on the water...
IMHO, the problems with this picture are:
1. The reflection looks as though it is just a blurred version of the top half, it looks "photoshopped". This needs some darkening on an adjustment layer to look "natural".
2. I think you have not quite got the right depth of field.
3. Mainly composition. If you had walked further round to the left and got the shore curving in from the bottom left of your picture it would have drawn the eye into the picture, and helped differentiate between the hill and reflection.
Anyway, just my two penneth...
Sent from my iPhone using Tapatalk