The Iron Trialfrom the Cinquain, the Second Principle of Magic
Water is a transparent fluid which forms the world's streams, lakes, oceans and rain, and is the major constituent of the fluids of organisms. Water is liquid at standard ambient temperature and pressure, but it often co-exists on Earth with its solid state, ice, and gaseous state, steam (water vapor) at high temperature. It also exists as snow, fog, dew and cloud. In terms of magic, water can be utilized as an element; mages can draw their power from it and manipulate it.
The ability to manipulate water allows mages to move, pick up and take hold of water without spilling it, and even shape it. Parts of bodies of water can also be controlled—an example would be using water magic to stay balanced on a piece of log in water. Mastery and training with water magic can also give a mage extraordinary abilities such as swimming "like they're fish" or making giant whirlpools. Water is also presumably an element involved with ice used in magical conditions. And like all magical elements, a mage can call on less evident powers from the water element: it can be used to heal.
The water in the underground river of the Magisterium can be coaxed to take boat riders where they need to be, particularly through the elementals found in the water which were often easily persuaded by a small amount of power to do a mage's bidding. The water in the Butterfly Pool, fed from a spring aboveground, is heavy with minerals that make it excellent for smithing weapons.
Fire is the counterweight, or opposite, of water. If a mage were to feel like they were being drawn into the fire element, they can try to reach for said element by performing water magic to save there soul, and vice versa. This also means that fire and water is most effective against each other and can be used when fighting respective elementals.