Secret Water is the eighth book in Arthur Ransome's Swallows and Amazons series of children's books. It was published on 28 November 1939.

This book is set in and around Hamford Water in Essex, close to the resort town of Walton-on-the-Naze. It brings the Swallows and the Amazons together and introduces a new group of characters, the Eels and the Mastodon. Ransome used to sail to Hamford Water, an area of tidal salt marshes and low-lying islands, in his yacht Nancy Blackett. He set the book here to offer his characters new opportunities to explore and make maps in a different landscape