I can't get rid of the thought that Mellville was describing something else than just a whaling advanture.
Maybe the boring parts are exactly the parts where Mellville by using analogy explains something else.
The book is about a journey or an expedition all right, but what kind of journey? Is it only an advanture book?
I wonder why Mellville would write a book which became responsible for his drop in popularity at the time, and why he didn't even try to be popular again. Maybe he didn't know what he was doing writing the book. But then again one can read in Wikipedia aobut him: "In his later life, his works were no longer popular with a broad audience because of their increasingly philosophical, political and experimental tendencies."
Maybe he knew exactly what he was writing; maybe we can't read between the lines as he intended for us to do.
What is/was he trying to tell us?