Underwater Temple of Poseidon's Heart

Publisher's blurb:

Glistening light filters down through water, landing on metal grids and stone walkways in this underwater temple. Further in, a shattered blue crystal heart glows from where it lies on cold stone, pinned down by chains held by angelic statues.
Fish dart between red, blue and white coral growing off the walls. Next to them larger-than-life stone faces stare across the temple, while two towers stand watch at the entrance. Large tone steps lead inside, to stone platforms connected by metal grids—some of which are overgrown with more coral.