First, Ahab, who is actually less of a presence in the novel than I expected, and whose motivation was surprisingly easy to understand. It's all in this passage below:
Hark ye yet again—the little lower layer. All visible objects, man, are but as pasteboard masks. But in each event — in the living act, the undoubted deed — there, some unknown but still reasoning thing puts forth the mouldings of its features from behind the unreasoning mask. If man will strike, strike through the mask! How can the prisoner reach outside except by thrusting through the wall? To me, the white whale is that wall, shoved near to me. Sometimes I think there's naught beyond. But 'tis enough. He tasks me; he heaps me; I see in him outrageous strength, with an inscrutable malice sinewing it. That inscrutable thing is chiefly what I hate; and be the white whale agent, or be the white whale principal, I will wreak that hate upon him. Talk not to me of blasphemy, man; I'd strike the sun if it insulted me. For could the sun do that, then could I do the other; since there is ever a sort of fair play herein, jealousy presiding over all creations. But not my master, man, is even that fair play. Who's over me? Truth hath no confines.The whale to Ahab represents that "outrageous strength" and "inscrutable malice" that built a universe which inflicts arbitrary punishment on its sentient inhabitants. His crusade is against a God that has left an imperfect creation for his creatures. And it's not the punishment, so much as the fact that its arbitrary, that incites Ahab's fury. This questioning of divine order is why the voyage is impious and blasphemous. Not only that, but Ahab is actually unsure if natural evil is animated by a hidden rational power – perhaps it's just arbitrary. It doesn't matter, fair play demands that Ahab fight the imbalance. The final part of the passage is a bit garbled, but it suggests that Ahab questions the notion of fair play as well, the "Truth" trumps it. But he reels himself back from such doubts, and goes on to address Starbuck's defiance.
The adjective that keeps cropping up with Ahab is "monomaniac", which serves as a useful contrast with the narrator and reigning principal of the novel, Ishmael. While Ahab's quest for vengeance supplies the propulsive narrative force of the novel, this keeps getting destabilised by the circling centrifugal nature of Ishmael's narration. Moreover, while Ahab supplies a singular reading of Moby Dick's significance, Ishmael's digressions concoct a heap of different allusions and conclusions bolted on to the whale, a lot of them (to me anyway) quite trite and unfulfilling. He is a mind at play, unbound by ideological or linguistic orthodoxies, delighting in everything around him (and mischievously undermining the veracity of his account). I think Melville didn't have to add that final epilogue explaining how Ishmael survives. It would have completed his ascension to the all-embracing, omnipresent consciousness he was moving toward through the book – from the individualistic self-definition of 'Call me Ishmael' to being dissolved into the sea and becoming one with the story he is telling.
I found the tone of the beginning of the novel to be unexpectedly wry and whimsical. A welcome surprise, but Ishmael's subsequent lack of focus on the ship became very tedious. I was left wishing that many more chapters fell under the heading of "sundry mystifications too tedious to detail". Melville tests the reader's patience in deciding to demonstrate Ishmael's irreverence and sense of wonder so comprehensively. Large sections of the book left me restless. The experiment of living provided by Ishmael, while maybe more admirable, was not that much more attractive than the experiment of living provided by Ahab, and I don't think Melville intended the two to cancel each other out.