This is book number 8 in the Godine Storyteller series.
On an uncharted island, the Swallows discover mysterious footprints of a very large animal and a dangerous enemy as well, another group of children known as "The Eels." Thankfully, the Amazons are on their wayThe Swallows and their father, Navy Commander Ted Walker, planned a camping trip but father was called away on naval business. Instead of camping, Father sets them a challenge: use a dingy to explore and create a map of low-lying islands until he can return. As the children explore the inlets, coves, mudflats, and estuaries, they end up in a friendly "war" with The Eels just as Bridget, Titty, and Roger are trapped in the middle of a ford by a rising tide. Friendship, family, resourcefulness, and sailing, too: Arthur Ransome's Swallows and Amazons series has stood the test of time. More than just great stories, each one celebrates independence and initiative with a colorful, large cast of characters. Secret Water (originally published in 1939) is the eighth title in the Swallows and Amazons series, books for children or grownups, anyone captivated by a world of adventure and imagination, exploring and setting sail.