As usual, the Water Mill is very useful for a general boost to a budding city's growth and functionality, and even more so if it has Wheat and Rice nearby. Unfortunately, it can only be built if you founded the city on a River. Fortunately, with the present city growth rules, you have really good reasons to found cities on rivers!
Historical Context Edit
A water mill uses flowing water to turn a water wheel which is geared to turn other wheels for grinding, spinning, sawing or pounding all manner of useful things: grain, lumber, textiles, ore, metal, paper, and so forth. Most folk think of water mills as gristmills, those adapted to producing flour from grain … and that is certainly the earliest type found. The Greek engineer Philo of Byzantium (c. 280 to 220 BC) left an account of the workings of a water mill; the Roman Vitruvius offered the first technical description of building one, his an undershot wheel with a gearing device. So useful was the water mill that by the time of the Dark Ages, they could be found near every settlement, and (unlike a lot of knowledge that got lost) the Europeans managed to retain the essentials to construct and operate water-driven gristmills. Later on, steam power and electrical power replaced the water wheels, being more efficient and economic; today, only barbarians would use actual water mills to grind their corn and wheat.