The practice of Hydrotheurgy, based on the art of Hydromancy is one of the magical arts correlating to one of the most basic abundant of substances, that of water. Hydromancy is the arcane manipulation of magic to achieve effects based around water, cold and the ocean.
Water is necessary for life, and with each culture depending on it it is easy to see where a level of admiration could arise for it. Being able to control the very substance of water, the chilling bite of the cold or the adamant rage of the ocean lead earlier Hydromancers to a level of unrivaled power in coastal communities. In some worlds, especially those where the oceans cover most of the surface these beings could almost be considered godlike in their power.
These days, it isn't uncommon to find Hydromancers working on sailing vessels, in port towns or any job involving traversing large bodies of water. Some Hydromancers instead find work conjuring water in areas where there is none, bringing life to normally dead areas as they create and maintain these magical oases. Some even use their mastery to explore deep places that most could only dream to reach.
One of the initial four magical arts taught during the first construction of the magical schools in the First Age, Hydromancy further developed once those from across the worlds finally had a place to meet and communicate about their art. It wasn't long until those with spells of cold, mastery of the ocean and general Hydromantic practitioners were able to create a unified school of magic that would become one of the most commonly practiced across the planes.