Mau is a boy just finishing his initiation to manhood, leaving his soul behind on a small island - when he canoes back to his home - Nation - he gets a man's soul. But on his journey he, and the rest of the world, face a tsunami that wipes out the world as it exists. He gets back to his home (by luck) to find it completely destroyed and he has to carve a life for himself.
Meanwhile the daughter of an ambassador is the only survivor of the wreck of the Sweet Judy - which also wrecks on the Nation. Setting the scene for a clash of cultures when the trouserman girl Daphne and Mau meet.
This is a wonderful exploration of the human spirit, the way individuals cope with disaster, rebuild their lives, their civilisations and work out their new societies and their belief systems; which makes it sound deadly dull, and of course it is far from that containing many flashes of Pratchett humour and slightly cock-eyed logic. The period is Victorian and the world similar but not entirely like our own - I believe the tsunami is probably based on the fallout from the Krakatoa eruption. Includes some wonderful and readable scientific deduction - Daphne is a science devotee. Read it for fun, and let it make you think too.