The Angels Will Not Care

Description for THE ANGELS WILL NOT CARE by John Straley:

Enter Sonny Walters, of the Great Circle Cruise Lines, whose first-class ship has a first-class problem. It seems there's some controversy surrounding the ship's doctor. More than that, Mr. Walters won't say, except that he will give Cecil Younger free rein -- as well as free passage and an unlimited bar tab -- to get to the bottom of the unsavory rumors.
All Cecil expected to find was an incompetent doctor and the excuse the cruise line needed to fire him. Instead what he finds is a lot worse and suddenly Cecil is out of his depth, far from home, and thinking of only one thing - survival.

This is #5 in Cecil Younger Series.