For the trip, John is Captain of the sailboat Swallow, Susan’s Mate, Titty’s Able Seaman, and Roger, the youngest, is Ship’s Boy.
Since they call themselves the first white explorers to the area, most of the other people, including Mother, are "natives," but there are also some "savages." And the four Walker children are sure the man on the houseboat is a retired pirate, Captain Flint—he has a parrot and a cannon, after all.
But they never expected to encounter a sailboat that’s flying a skull and crossbones.
They’ll declare war on the pirates aboard the Amazon, attack Captain Flint, and hunt for buried treasure.
Since this book was first published in the 1930s, it has a slower pace than modern books, but it gets more exciting by Chapter 8—and if you read to the end, you’ll find out about the wars they fight, about the treasure, and about who’s forced to walk the plank!
Swallows and Amazons by Arthur Ransome (David R. Godine, 1985; originally published 1930), 351 pages.
If you want more Swallows and Amazons, there are 11 and ½ other books in the series, all available from Arthur Ransome's author page on Amazon.com.