A neighbour of mine has been trying for the past two years to do just that - he lives in an old mill which has a leet, with a working water head of around three feet (albeit a slow flow), powering an enclosed Pelton wheel coupled to the working parts of an old two-stroke generator.
He's chuffed to bits because he can light a 60 watt bulb in his shed/office. Somehow I don't think it's going to remove his dependency on the grid!