Among the Lost — Armand Croft

While on a Caribbean cruise Andrew MacCrimmon uncovers a plot by a man named Thomas Padgett to kill his wife and replace her with her twin sister. When MacCrimmon confronts him, Padgett, a giant of a man, carries him bodily to the rail and drops him in the ocean. An old man who witnesses this tosses MacCrimmon a life ring. After floating two days in the Caribbean he is rescued near death by Cuban fishermen. After enduring nine months in a Cuban prison suspected of being a Yanqui spy, MacCrimmon, skeletally thin, severely malnourished, and worm infested, escapes to Florida where he learns his wife, Karen, thinking he is dead, has remarried. Before he rebuilds his life, MacCrimmon has two goals: to regain his health and fitness; and to find and kill Thomas Padgett.