Perhaps Melville's intent can be discerned by understanding Ishmael's role in Moby Dick. Besides being the vehicle of ushering in the story and problem of the narrative, Ishamel is also the voice of reason and the phiosophical interpreter of Melville's message. Ishmael empathizese with Ahab's quest; understands Ahab's perspective that love exists on one plane of existance, while evil exists in the "terrible unseen"; perceives the paradoxes that (1) unity is borne of multiplicity and (2) oneness is generated by diverse differences. From this we might infer that Melville's intent is to present the power of the unseen and these paradoxes of living.
The themes in Moby Dick are numerous and range from the religious theme of God and religion to appearances versus reality, with others, like the individual versus nature, good versus evil, the feminine versus the masculine, and choices and consequences, included as well. This format cannot cover all these but can allow for examination of one.
The theme of God versus religion pits humankind's freedom to choose against providentiality or fate that seems to dictate what is inevitably determined. Ahab cries out questioningly against the prescribed role of God and religion and determinism in the world. This also pits him against Starbuck, as Starbuck is the emblem of belief in the power of choice resisting the inevitable:
Oh, my Captain! my Captain! noble soul! grand old heart, ... Away with me! ... let us home! ... - this instant let me alter the course! How cheerily, how hilariously, O my Captain, would we bowl on our way to see old Nantucket again!
Ahab is engaged in a fight against "some invisible power": the pursuit of Moby Dick is emblematic of this fight.
Is it I, God, or who, that lifts this arm? But if the great sun move not of himself; but is as an errand-boy in heaven; nor one single star can revolve, but by some invisible power; how then can this one small heart beat; this one small brain think thoughts; unless God does that beating, does that thinking, does that living, and not I. ... [We] are turned round and round in this world, like yonder windlass, and Fate is the handspike.