Annalie and Will live with their father, Spinner, in Lowtown, one of the shabbier parts of a city in Dux, in a world struggling up from a worldwide flood connected with attempts at fixing climate change disasters. Spinner is a mysterious figure. They live "off the grid."
When Spinner suddenly has to flee from pursuers he has expected for a long time, the twins also must escape, Annalie bringing her friend Essie from their boarding school, where she had a suspicious visitor, Beckett, soon after her father's disappearance. Beckett had asked some questions that made her conclude he was up to no good. The children, who had spent a lot of time sailing with their father, know how to sail very well. They have to steal back the boat, which has been impounded by the Admiralty, the world's rulers, and set sail to find their father, using a few clues they have picked up. One more member is added to the crew, Pod, a former slave rescued from a rock where he was marooned. Then the adventure proper begins.
And it is quite an adventure, or a series of adventures, from talking apes(left over from an experiment in making animals talk)to a cannibalistic religious cult. They're travelling through an archipelago of islands that range from dead to tropical, so they might find anything along the way.
There is a definite lesson on what climate change might lead to, not to mention who might rise to the top - in this case, the Admiralty, which began as a way of getting the world through the crisis and ended up staying in power.
The characters are good, each of them contributing their knowledge to the quest. Annalie is the intellectual, who remembers things - and the only possible navigator. Will steers the boat and does most of the repairs when needed. Essie, the rich girl with no special sailing skills, offers to cook - and, at one stage, becomes ship's medic because she saw all this stuff on a TV series. Pod can't swim, but has plenty of support skills to offer, and knowledge of pirates.
I did wonder how the villain, Beckett, was able to track the fugitives so easily, something not explained by the end of the book. He simply turns up right under their noses whenever they're feeling safe. Not on a following ship, though there are those, but right there, ready to capture whichever of them is in a street, getting supplies along the way. The children wonder briefly about it themselves at one point, then don't discuss it again. Perhaps it will turn up in the next book, though I suspect not.
Still, suspension of disbelief should help the reader get on with enjoying the book, which is basically a road story with islands instead of towns on the way.
There's also a distinct flavour of Jules Verne, especially Captain Grant's Children aka In Search Of The Castaways, a Disney film of the 1960s. Well, why not? Perhaps children who read this might try Verne next.