The Second Heart
A typical human has an innate affinity with water, the only magical substance in the Oceanic Eon. All magic is a variation of manipulating water: altering surface tension of water to walk on it, boiling water in a cooking pot, turning the wheel of a watermill, etc.
The typical human is able to do this because of an organ called the second heart. It resides on the opposite side of the first heart, the heart that deals with blood. While the first heart circulates blood throughout the body, the second heart derives magical power from the water stored in it and circulates it in a similar way.
The second heart also converts the first heart's blood to water, and its own water to blood. This is an involuntary process. At birth, the blood-water conversion factor is about 1:1. This conversion factor depends on the strength of each heart, which changes with exercise over time. As magic is used more, the second heart becomes more efficient at deriving water from blood. As physical skills are used more, the second heart becomes more efficient at transforming water to blood.
Some humans, out of necessity or habit, use magic more than their own bodies can bear. When their thirst becomes severe enough, their first and second hearts fight for control over the body's remaining water. If the first heart prevails, the second heart dissipates into water and cannot be replaced. If the second heart prevails, the first heart begins to die, and death will occur unless the human consumes enough water or the heart is replaced.