This is book number 8 in the Godine Storyteller series.
Friendship, 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.
In Secret Water, the five Walker children are left on a "desert island" by their parents with provisions for a long stay and a blank map to fill in. As the children explore the inlets, coves, mudflats, and estuaries they end up crossing paths with enemies (another clan named "The Eels") but friends as well (who else but the intrepid "Amazons?")
The entire Swallows and Amazons series is for children or grownups, anyone captivated by a world of adventure and imagination, exploring and setting sail.