Books like Slack tide by George Harmon Coxe

The house was on a low-lying island just a hundred and fifty feet across the inlet from the MacLaren Boatyard. A year before, it had been bought by Oliver Kingsley—notorious for his night-club escapades and unsuccessful marriages—and remodeled as a summer home. Contrary to the natives' expectations, he had done nothing unusual to it, but they had noticed that the windows of the corner room at the rear of the house were kept continually shuttered. Harry Danaher, the captain of Kingsley's cruiser and general man-about-the-house, couldn't—or wouldn't—say why. It was up to the girl who swam out of the water one night to supply the answer—an answer that put both young Donald MacLaren and the girl in jeopardy from the police and danger from person or persons unknown. Before they had seen it though, death was to come twice—once to the good and once to the bad—and the seeds of fear and hatred sown by one who had sought only power were to produce their expected harvest.

First publish date: 1959