We have a thrilling rescue, the high seas, and a whole new country. Then, on top of that, a return to a home country that doesn't want our main man back and readily tosses him in jail to prove it. This book is all about high stakes, physical drama, and new places—all of which means we're definitely in the adventure genre.
Okay, this one is easy. Here's why: John Mung a.k.a. Manjiro? A real dude. The story of him getting picked up by Captain Whitfield and his crew, of him going to America and then back to Japan? Yup, all of it's real, too. But it's not told by Manjiro himself; the story is told through a third-person narrator created by Margi Preus. So the book is basically a biography.
That said, a warning: the book calls itself "a novel inspired by a true adventure on the high seas." What does that mean? It means the novel—a piece of fiction, ultimately—is an imagined biography. The author has tried to fictionalize or make up what she thinks Manjiro might have thought and felt while experiencing these major events. And for this reason, you can't take everything you read in the book as factual and true, especially when it comes to Manjiro's words, thoughts, and feelings.
Manjiro is a young guy, bound to his family and to Japan, at the beginning of the book. But pretty quickly, he learns to go his own way, and this means moving away from his friends to go with Captain Whitfield to America. It's huge. He goes out on his own, leaving everyone familiar behind, in order to experience a new country and way of being. How could he not mature into an adult, given all of that experience? Manjiro's definitely grows up over the course of this book.