High Water by Douglas Reeman
With his own boat, the motor yacht Sea Fox, former naval officer Philip Vivian had hoped to earn a living free from the petty restrictions of everyday life, close to the sea he loved. Now, however, his dream is threatened by financial difficulties. So when a profitable, if legally dubious, proposition is put to him by an old naval comrade in arms, Vivian is willing to listen. But what starts out as a harmless adventure soon turns into something altogether more sinister. And Vivian finds himself trapped in a treacherous web of violence and crime, dangerously torn between his stubborn sense of past loyalties and his duty to a society he has always despised.