Drowned Wednesday - Garth Nix

On the third day, there were PIRATES! Arthur Penhaligon finds himself on an adventure that will pit him against pirates, storms, explosions and a vast beast that eats everything it encounters. Will our unwitting hero be able to find the third part of the Will and claim the Third Key?

No rest for poor Arthur Penhaligon. As Grim Tuesday ends, he discovers a square of stiff cardboard under his pillow, gilt-edged and inscribed with the following words:

LADY WEDNESDAY
Trustee of the Architect and Duchess of the Border Sea
has great pleasure in inviting
ARTHUR PENHALIGON
to a Particular Luncheon
of Seventeen Removes

Transport has been arranged
RSVP not required.

It's an invitation he cannot refuse. From hospital room to the high seas, Arthur finds himself on an adventure that will pit him against pirates, storms, explosions of Nothing-laced gunpowder, and a vast beast that eats everything it encounters. Through it all, he is drawn deeper into the central mystery of the House. Arthur must find the third part of the Will and claim the Third Key - not just for himself, but for the millions (if not trillions) who will suffer if he doesn't.

The first step? Surviving life aboard ship on the Border Sea.