well from what i been reading I think, Madara was the Third Mizukage this was the person who When a group of sword-using shinobi slaughtered the whole Kaguya clan, the Third was so impressed that he named this group the Seven Swordsmen of the Mist. And that is how Kisame knows him..
The Mizukage now the Fourth Mizukage what we know about him is he rules the village with a iron fist and corruption then disbanding the Seven Swordsmen which is why Zabuza, being the former, albeit most recent leader of the Seven Swordsmen and former ANBU squad captain tried to kill him