Malachite. The Paladin Cycle, #1

Marion Casterline is the highwarden of Malachite, an ancient, beautiful city floating in a shallow sea. In the aftermath of a brutal gang war, there is finally peace in the city, and new life every year through the sacred ritual of Aequora. Through Aequora, exiles, outlaws, and orphans can become citizens of Malachite. This ritual is vital to the city's survival, because Malachite is populated only by males.
Jean Rivard grew up in the Zanzare slums at Marion's side. As boys, they were branded into the violent Teschio gangs ruling the criminal underworld of Malachite. Known in the slums as the Prince, Jean became a spy, an assassin, and Marion's lover. As men, they worked together to destroy the Teschio and crown Kon Sessane as magestros of the city, only to drift apart after the battles were won: Jean to the Black Keep, Marion to the grand halls of the Consolari.
When Marion announces his engagement to Kon's son, Jean is hurt and resentful. Marion is leaving him and their past behind in every possible way. Marion also believes that he's starting a new life, but when a charismatic rebel leader kindles a revolt in the slums, he realizes that the only man who can prevent war from devouring the city he loves is his very own prince.