Abyss Azure no Zainin

The ocean covers about 70 percent of the earth's surface—a whole world, yet unknown to humanity… And in its depths, a nation, thousands of meters beneath the waves. There, the mermaid Jo has been whiling away her days in peace. But that tranquility is shattered when her friend Ryuu is locked away for breaking the gravest of undersea laws: that humans must never know of the world below.
But why would Ryuu let the secret slip? And who did she tell? The curtain rises on Jo's adventure to save her friend—and on her love..!?
(Source: Yen Press)