Dark Waters — Susan Rogers Cooper

The Kovaks are all at sea when a family trip goes deadly wrong . . .

When Milt Kovak wins a cruise for four to Puerto Rico, he takes his family -- wife Jean and son, Johnny Mac, plus Johnny Mac's best friend, Early Rollins. But the trip takes a turn for the worse when Johnny Mac and Early are caught stealing on the ship, and the body of the boy who bullied them into it is found dead on the top deck. Milt and Jean team up with the ship's security officer to try to find the killer, before they strike again .