Gwen, in Green (Paperbacks from Hell)

Young couple Gwen and George fulfill their dream of buying a secluded private island. However, they soon uncover a dark, centuries-old secret tied to a past tragedy in the island's waters. As George observes unsettling changes in Gwen's personality, people around them begin to mysteriously vanish. He must confront whether the island's secret is claiming new victims, or if his wife is transforming into something terrifying.