Vault of Sunken Riches

Publisher's blurb:

Glistening light filters down through water, landing on scattered gold coins and other precious treasures—though they are locked behind thick doors. Outside, fish dart between red, blue and white coral growing off the walls. Next to them larger-than-life stone faces stare across the underwater fortress, while two towers stand watch at the entrance.
Large tone steps lead inside this structure, to stone platforms connected by metal grids—some of which are overgrown with more coral.