Literally “water charmer”. Hydromancers are sorcerers who exclusively use magic to control water in all its forms. During the Sundering, Hydromancers kept vast amounts of fresh water centered on the Islands to drink for those who survived. They also used powerful spells to provide an inertial dampening effect within the life forms on the Islands during the explosion of the planet. Without that effect, most animal life would have been squashed flat as the Islands accelerated away from the exploding center of the planet.
After the Sundering the Hydromancers channeled rivers to run from the central seas off the edge of the Islands to run down to the bottom tip and funnel back up through rock, purifying it before it reached the central sea again. After the War of Wind and Fire, the water mages began to be seen as second class citizens, river rats who were usually dirty, moldy, and lazy. They can be found wherever there is water and usually charge a pittance to ferry people along the waterways crossing the Islands.