Secret Water

Secret Water is the eighth book in Arthur Ransome's "Swallows and Amazons" series of children's books. It was published in 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 sailed to Hamford Water, an area of salt marshes and low lying islands in his boat "Nancy Blackett". He set the book in this tidal place which offered a new setting for his characters and chances to explore and map the area.
Plot summary.
The Swallows mean to sail in the "Goblin" to Hamford Water and camp with their father, but he is called away on naval business. Instead he maroons them with a small boat (dinghy) on an island. Before he leaves, Father gives them a basic map of the area they call Secret Water and suggests they map the area before he returns to pick them up.
For a surprise, he has arranged for the Amazons to come down from the Lake District and join them with another dinghy.
They see some mysterious footprints which turn out to belong to the Mastodon, a local boy. He mistakes them for the Eels, another family who camp in the area regularly. Later the Eels arrive and are initially hostile before they settle down for a friendly war.
It seems that due to the distractions of war and being cut off by the tides, the chart will not be completed. However early in the last morning two separate groups of children complete it.