Published in Super-Detective Library #59 - August, 1955.
"Deciding that it was about time he had a vacation, the Saint and his friend, Don Maddon, hired a launch and set off across the blue waters of the Gulf of Mexico to find a quiet spot to do some fishing." They end up near a group of islands just off the coast of Florida where Doctor Harris and his daughter, Emily, and partner Clegg were experimenting with the Doctor's latest invention - a detectosope, in the hope of finding sunken gold.