Guardian's Keep — Eric Douglas

Dr. Francesca "Frankie" DeMarco, a beautiful Italian archaeologist, is searching an underwater site on the Adriatic Coast of Italy. Chasing stories of a mysterious group of Guardians, she runs into roadblocks. Someone is trying to keep her from discovering the truth.  

To unlock the mystery of the Guardians, Frankie and Mike Scott have to overcome a group of delusional fanatics. If they are successful, they might just find a religious artifact lost when the Romans overran the Temple in Jerusalem: the Breastplate of Judgment, worn by the Jewish high priest when he went before the Ark of the Covenant to talk to God.

If they aren't successful, they could be entombed for the rest of their lives.