Mizukage

The is the leader of Kirigakure no sato. As such, their word holds sway over all political and military issues that present themselves regarding the village and its inhabitants.

Madara Uchiha
The only named Mizukage that has been introduced so far in the series is Madara Uchiha, though it's unknown when he gained the title of Kage and how many Mizukage came before and after him. It has been confirmed that Madara is not the current Mizukage. Nothing has been revealed about his time as the Mizukage.

Current Mizukage
In the Eight-Tails arc, the current Raikage of Kumogakure demanded a meeting of the five Kage to discuss the problem of the Akatsuki. Whoever the current Mizukage is, they are likely to make attendance.

Coup d'etat
Several years prior to the start of the series, Zabuza Momochi attempted to assassinate the Mizukage and stage a coup d'etat. He ultimately failed and became a missing-nin. It is unknown if his target was the current Mizukage or a previous one.

מיזוקאגה