This is book number 3 in the Swallows-And-Amazons-For-Ever! series.
Arthur Ransome's Swallows and Amazons series has stood the test of time. Ransome is not only a great storyteller, writing from first-hand experience, but each story celebrates eternally valuable qualities of practical knowledge, independence, and initiative. The twelve books are for children or grownups--anyone captivated by a world of sailing, adventure, and imagination.
The third book in the series, Peter Duck takes intrepid explorers John, Susan, Titty, and Roger Walker and fearsome Amazon pirates Nancy and Peggy Blackett onto the high seas. Under the command of the infamous Captain Flint (Nancy and Peggy's Uncle Jim), the children brave a real-life pirate and his cutthroat crew, sharks, and the ravenous creatures of Crab Island in the search of buried treasure.