This school does stuff with water and other liquids. Moving and storing water, especially preventing evaporation of such, is pretty useful in basically any society. Along with geomancers, they can manage irrigation, as well as aqueducts and sewers. And, of course, stopping fires. As such, hydromancers not engaged in academic activity are usually found in the civil service. Hydromantic combat mostly amounts to high-speed jets of water. Ice can be manipulated as well, but hydromancers cannot control temperature, so the ice must already be such. However, the manipulation of liquids other than water can have interesting applications in combat, such as throwing acid on someone's face, or - with great effort and skill - stopping blood flow.