A fish tale, of sorts

In her alternate histroy science fiction novel The Moon and the Sun, Vonda N. McIntyre portrays the moral struggle of Marie-Josephe de la Croix. Marie-Josephe is the sister of Father Yves de la Croix, the natural philosopher of King Louis XIV of France. Father Yves has just returned from a sea voyage with the possible source of immortality, a living sea monster. As the story unfolds, Marie-Josephe must decide between the powers of her world and her own conscience. What will she choose?