Troubled Waters by Sophie de Mullenheim

The fourth book of the Shadows of Rome series. Maximus, Aghiles, and Titus try to stop a gang selling water on the black market in pagan Rome. When Maximus discovers that more than half the water piped into Rome is stolen and sold on the black market, he needs proof in order to expose the traffickers, especially when everything seems to be pointing at a Christian conspiracy. With the help of his two friends Titus ans Aghiles, Maximus is once again ready to face peril without any idea of what truly awaits him.