I think there are several reason why I found the Great American Novel boring. The first, and probably least important reason is that Melville spends a long time just setting the story up. Several chapters are devoted to background, making it hard to get into the story since initially there isn't really one. The second is probably an accurate reflection of a whaler's life - long stretches of fairly hum-drum sailing punctuated by bursts of frantic excitement as a whale is spotted, pursued, killed and then processed. The third, and most frustrating for me as a reader, is that the author frequently interrupts the narrative flow with lengthy passages on the history of whaling, encyclopedic entries on different types of whales, and seemingly unrelated accounts of events that occurred on other whaling boats. Some background is fine, even necessary, but Melville provides way too much - at least for my taste.
As I read I wondered what the novel would have been like had it been told from Ahab's point of view. He was the one driving the action, the one obsessed by revenge, and yet he is mainly just a shadowy figure. I suspect I might have enjoyed the story more had there been more of Ahab in it. He was certainly central in some of the scenes - such as his meeting with Captain Boomer - which I enjoyed and found most compelling.
One positive for me was the the quality of the writing. Gems like
The warmly cool, clear, ringing, perfumed, overflowing, redundant days, were, as crystal goblets of Persian sherbet, heaped up - flaked up, with with rose-water snow
It was a beautiful, bounteous, blue day; the spangled sea calm and cool, and flatly stretching away, all round, to the horizon, like gold-beater's skin hammered out to the extremest
helped make up for having to slog through many pages that I found dreary in the extreme.