Underwater Temple of the Frozen Heart

Publisher's blurb:

Glistening light filters down through icy 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—though it waits above the waterline, sealed behind thick doors.
Ice coats rocks and metal walkways throughout this temple, while fish dart between them. Larger-than-life stone faces stare through the frigid water across the central chamber, while two towers stand watch at the entrance. Large tone steps lead inside, away from the cool, rocky floor.