Tomcat and his friends have a paddlewheel steamboat and travel up and down the Mississippi River carrying mail and performing impromptu shows at the river towns on their route. (Of course, I had to destroy civilization to arrange it for them.) They have a strict rule against carrying passengers, which is broken a chapter and a half into the book. The rest of their adventures illustrate why that was a good rule.
Published as a YA novel (Harcourt, 1992), these days it might even be considered a middle-grade book. I grew up near Winona, Minnesota, on the upper Mississippi River. Back then, there was a decommissioned steamboat, the Julius C. Wilkie, turned museum there. I drew on my visits to that museum when imagining the River Rat.
In addition to RIVER RATS, they also have WHEN THE KING COMES HOME
(What if a young woman apprenticed to a fantasy Renaissance painter in a fantasy kingdom had adventures that involved rescuing the local equivalent of King Arthur from that most dangerous of villains, a rogue librarian? Although this fantasy novel [Tor Books, 2000] was written for adults, the content is YA. )
and earliest of all, THE SERPENT’S EGG
Skulduggery at the royal court in a fantasy kingdom. Who will defend the aging queen? Gallant ladies, honest knights, wily minstrels, and bossy sisters — but does the queen need defending? Originally published in 1988, this is a very early work.