The Mizukage (水影; ; Literally meaning "Water Shadow"; ) is the leader of Kirigakure no sato. As such, their word holds sway over all political and military issues that present themselves regarding Kiri and its inhabitants.
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's not unreasonable to guess that he was the First. It has been confirmed that Madara is not the current Mizukage. Nothing has been revealed about his time as Kiri's leader, with the exception of him having a relationship with Kisame Hoshigaki, one of the Seven Ninja Swordsmen of the Mist.
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.
Several years prior to the start of the series, Zabuza Momochi, another one of the seven mist swordsmen, 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 Madara, the current Mizukage, or a previous one.