The Tide Knot, in reference to the book rather than the object, is the second in the Ingo series by Helen Dunmore.

In this book, Sapphire and Conor unravel the secrets of the ocean, revealing the reasoning of Mathew Trewella, Sapphire and Conor's father and the importance of the Tide Knot. As the Tide Knot gets loosened, the Mer are excited for the reign of the underwater world Ingo to break loose and swallow the Air.