Scout: The Sailing Sleuths by Piet Prins
While Tom and his two friends are making plans for a vacation, Carl's father announces that he has acquired a sailboat, as part of a business deal. The three boys see the chance for a very unusual holiday.
Their peaceful sailing trip is sabotaged when they run into a gang of notorious carnival followers who put on sidewalk shows as a cover for more profitable sidelines. After a confrontation with the gang, the three sailors and Scout find themselves in trouble with the police. Their attempt to clear themselves leads to a wild chase through rivers, canals, and lakes. When at last the boys come face to face with the gang and its ruthless leader, they are stranded on an island in the middle of an isolated lake. In the showdown they are all alone — except for Scout.