Don't crop the image because of a "rule," crop it because you (and only you) feel it betters the image! At the end of the day, if the image doesn't speak to what you meant to capture, then its not worth its weight in pixels.
I cannot stop looking at the halo-ish thing over the trees on the left. I'm not sure what it is, but something feels off with the composition. Maybe it's all the empty water - you need a cruise ship there!
agreed. I think of it more like your image might be boring if you centre the subject, so think about putting it somewhere else ... calling it a rule makes me want to break it.
I think I would like your pic better if about half the water was cropped out.
Looks like the lake level is lower than usual. Is it a reservoir or just low this time of year?
Thanks for the comments everybody. @bc_steve the shot was taken in august last year (2012), so it might be a bit low because of all the heat, but yes it is a reservoir. The lake is called Oasa Lake and it is situated at 1255 m altitude. It is situated on the Transalpina road (one of the highest roads in Romania).