The Water Mage uses an arcing bolt of magically charged water to repel enemies (it looks like a swirling icicle with purple spots). Like most mage attacks it does 5 points of damage. He doesn't have any other means of attack, so circling and attacking him would be the best strategy in case of a fight. In this case a Water Mage is a weaker foe than a Hammerite Priest as the mage does less damage and has less health and higher fleeing point. His strength however comes when he attacks in combination with a guard or a mage, especially an Earth Mage. In both cases it will be hard for the player to reach the mage and Garrett will be an easier target for his water bolt.
Following the general rule in the Thief Universe that the more magical and supernatural enemies are usually immune to their own attacks, Water Mages are immune to the Water Bolts. But strangely they still drown in water like the other humans, even though some of their lore books, such as The Book of Coral and Scale, describe them "living" and breathing underwater.