Osmosis seems very counter-intuitive to me. I will try to explain my doubt with the following thought experiment:
Consider a huge container divided into two by a semi-permeable membrane. We add equal levels of water on both sides. On one side, dissolve some sugar. After some time, there will be a net movement of water to the sugar containing side of the container. This will increase the level of water on one side and decrease it on the other. If we create a huge hole in the membrane (large enough for both water and sugar molecules), then water will rush from one side to the other, equalizing the levels. If we were to somehow place a turbine at this hole, we could generate quite a bit of energy. Where did this energy come from?