The Leviathan by Rosie Andrews

Norfolk, 1643. With civil war tearing England apart, reluctant soldier Thomas Treadwater is summoned home by his sister, who accuses a new servant of improper conduct with their widowed father. By the time Thomas returns home, his father is insensible, felled by a stroke, and their new servant is in prison, facing charges of witchcraft. Thomas prides himself on being a rational, modern man, but as he unravels the mystery of what has happened, he uncovers not a tale of superstition but something dark and ancient, linked to a shipwreck years before. Something has awoken, and now it will not rest.