Vitruvius, the godfather of Roman engineering, describes several pieces of technology that the Romans used for water power. Combining Greek technologies like the toothed gear and the water wheel, Romans were able to develop advanced sawmills, flourmills and turbines.
The undershot wheel, another Roman invention, rotated under the force of flowing (rather than falling) water, making it possible to build floating waterwheels to grind grain supplies. This came in handy during the siege of Rome in 537 A.D., when the defending general, Belisarius, solved the problem of the Gothic siege cutting off food supplies by building several floating mills on the Tiber to keep the populace supplied with bread.
Strangely, archaeological evidence suggests that though Romans had the mechanical expertise necessary to build all sorts of water-powered devices, they did so only rarely, preferring cheap and widely available slave labor instead. Nonetheless, their watermill at Barbegal (in what is now France) was one of the largest industrial complexes in the ancient world before the Industrial Revolution, with 16 waterwheels to grind flour for the surrounding communities.