Those fancy robes are not just for show.
The teeth and fingerbones of holy men and more
The Pool of (add the name of the God or Goddess of your choice) is well known as one of the greatest holy places of the world. It is said to be full of piles of gold and jewels, a place where wishes are granted, wounds are healed, prophets have dreams that come true, and suspected criminals are fairly judged. Pilgrims from the other end of the world have been known to travel for more then a year, braving the perils of disease, brigands and bad weather, just to reach it's holy waters.
The heroes are travelling the great seas by ship. One night when the seas are calm and everything seems fine their vessel is eaten by a great whale. The end? No. They wake in the belly of the animal, it is like a great cave. Full of wrecked ships and strange things to be discovered. Will they make it out alive?