One of the problems with great books is that people seem to think they have to like everything or there’s something wrong with them. Moby Dick has some really dull passages, I don’t think there’s any getting away from that. Melville was a marginal writer during his lifetime, he wasn’t someone like Dickens who knew his public intimately and was able to cater for them. He was an outlier and sometimes got things wrong, so that at times you feel he wasn’t writing for anyone but himself. That’s part of its charm, in an odd way, but it’s also a flaw.
Of course you’re entitled to your opinion but what possible value did that long digression about the Battle of Waterloo have to do with anything? It was pure authorial self-indulgence and added nothing to the story at all. I think he just ran out of ideas and trod water for a while just filling the page until he thought up something else.