Six Months with Cerberus by Naomi Lucas

Cyane was delivered as a newborn to an orphanage with nothing but a cryptic note from her parents to come to Sicily for an obscure celebration on her twenty-fourth birthday.
Years later, desperate to get to the festival on time to finally meet her parents, she’s tricked and dragged under the Ionian sea. Confused, fighting for her life, and near death she’s pulled from the water by a man. An ancient Greek warrior with frighteningly animalistic eyes.
Cerberus, son of the dragon Typhon, hundred-headed hound to Hades, Gatekeeper to the Underworld, Watcher of all the souls in Styx, and first shifter, senses an unauthorized mortal in Hades’s domain. And when that mortal tries to escape, he does what he’s always done best—stops them.
What he doesn’t expect is a beautiful human woman cowering beneath his blade. When she begs for his help to leave Hades’s realm, his loyalties are tested unlike never before.