Captain Ahab is obsessed. He is a monomaniac, bent on meting out retribution to a white whale. When we see Ahab's quest as a personal conflict between Ahab and Nature (or God) as represented by the whale and the sea, we can argue that Ahab gains some measure of victory in his battle.
Contained in Ahab's desire for revenge is also a refusal to allow God to be the arbiter of his fate. He will choose his own fate.
He will fight against fate, rather than resign himself to a divine providence.
That his fate will be to die in his confrontation of Moby Dick does not lessen the fact that it will be Ahab's choosing to make this confrontation.
In this way, Ahab does achieve some victory. It is a victory of free will.
In making a choice and sticking by it, he can be seen as valiantly exercising free will.
Ahab's will is never conquered or diminished even as he is destroyed by his obsession with revenging himself against Moby Dick.