Reef of the Dead

Publisher's blurb:

Sunlight glistens past rippling waves, landing like dappled rainbows on dour underwater buildings. Built into a dark reef, small circular dwellings stand, their rounded domes decorated in highlights of brass. Walkways are paved with tiles and spiralling walls, while seaweed clumps along the edges.
Different blue and gray sea life grows off the surrounding shelves, inhabited by various types of fish and a shark. Further down is only the blue of even more ocean, descending to unknown depths.