to recognize this particular whale if anyone spots him. It seems as though their chances of seeing this one whale in all the vast ocean are miniscule. And the beautiful gold coin nailed to the mast isn't going to make it any easier to find a particular whale. Moby Dick could be virtually anywhere between the Antarctic and the Arctic, and between Asia and the Americas--or perhaps elsewhere. Ahab may be looking for Moby Dick, but Moby Dick certainly isn't looking for Ahab. Here is how Ahab describes the whale:

"Whosoever of ye raises me a white-headed whale with a wrinkled brow and a crooked jaw; whosoever of ye raises me that white-headed whale, with three holes punctured in his starboard fluke--look ye, whosoever of ye raises me that same white whale, he shall have this gold ounce, my boys!"

Apparently a crewman might spot a white-headed whale, but it wouldn't be Moby Dick unless he had three holes punctured in his right fluke. Whales have big triangular tails, and each side is called a fluke. Ahab himself may have been responsible for those three holes.

Herman Melville probably made Moby Dick a white whale because that was the only way the sailors could identify him as Moby Dick. Melville may have then attached specialized symbolism to the color white in order to explain a simple, pragmatic literary expedient. Moby Dick had to look distinctive. If he symbolized evil, he should have been described as black--but black would not be distinctive enough, since whales are usually dark-colored anyway. Too much significance can be read into many things in fiction which are only devices to serve a practical purpose. This can lead to endless speculation. As Freud once said: "Sometimes a cigar is just a cigar." Melville obviously knew a lot about whales and whaling. He must have known that albino whales existed, although they must have been quite rare. If albino whales were common, then it would still be hard for the sailors to identify Moby Dick. But in the novel, as soon as someone saw a white whale, he knew it had to be Moby Dick. Imagine how hard it would be to find a single whale in all the great oceans of the world! 

The white whale, Moby Dick, is associated with both good and evil, with nature and with God. The whale symbolizes opposition (to Ahab) and mystery (living in the wild and dark sea). The whale may even be seen to represent the limits of man to control this wildness of the natural world.

As an aspect of nature, Moby Dick is aligned with the glory and beauty of nature as well as the danger and power of nature. 

The whiteness of the whale is given consideration in its own chapter.

In this particular chapter, Ishmael meditates on the strange phenomenon of whiteness, which sometimes speaks of godly purity and at other times repels or terrorizes with its ghostly pallor. The meditation leaves color references behind to become a general meditation on the nature of fear and the existence of unseen evil: “Though in many of its aspects this visible world seems formed in love, the invisible spheres were formed in fright.” (eNotes)

In addition to being associated with the forces of invisible chaos beneath the surface of the sea, the whale is strongly identified with God. The whale is both one of God's creatures in the novel and a representative for the very idea of God and the idea of a natural order with God at its peak/top. 

Ahab resists and rebels against the natural order, willing to go against any force that would take from him his own free will.

To Ahab it does not matter if the white whale is “agent” or “principle.” He will fight against fate, rather than resign himself to a divine providence. (eNotes) 

There can be many answers to such a question, enhanced only by the complex nature of the work.  One way that Melville's work succeeds as an allegory is through the examination of the relationship between individual and nature.  There is a very strong element of allegory or symbolic analysis present when analyzing how Ahab appropriates the natural world.  Similar to America itself at the time of Westward Expansion and Industrial growth, there is a clear parallel in how Ahab approaches the idea of that which lies outside his control.  His stubbornness and determination are both examples of his strong qualities, but also representative of absolute terror in that he never relents in his hope to control an aspect of nature, an element that lies outside his control and exacts much in the way of human cost.  Another allegorical relationship concerns the idea of pursuing dreams.  On one level, dreams animate and inspire Ahab, as his dream of catching this whale are the source of his action.  Yet, Ahab becomes victimized and crushed, literally, by the weight of his own dreams.  In the process, the novel reminds us how dreams have to have some level of control for we should always control them, as opposed to them controlling us.

The major theme of the novel that the symbol of the whale itself supports is the limits of human understanding. At the beginning of the novel the narrator tries to discuss the topic of whales and what they represent, but his task is made impossible by the multiplicity of meanings that whales have had through the centuries and also the way that man's knowledge of them is so limited, because they cannot see beneath the sea. In the same way, the novel as a whole points towards the vast realms of understanding and knowledge that humans are not aware of.

This is highlighted through the whale in the way that so many meanings are attached to it. For the sailors, the whale is a symbol of their fears and the danger they face as sailors. For Ahab, of course, the whale represents something completely different:

All that most maddens and torments; all that stirs up the lees of things; all truth with malice in it; all that cracks the sinews and cakes the brain; all the subtle demonisms of life and thought; all evil, to crazy Ahab, were visibly personified, and made practically assailable in Moby Dick. He piled upon the whale’s white hump the sum of all the general rage and hate felt by his whole race from Adam down; and then, as if his chest had been a mortar, he burst his hot heart’s shell upon it.

The fact that the whale becomes a kind of repository for the feelings, fears and neuroses of other characters suggests again the theme of the limits of human knowledge, as characters seem to push on to the whale their fears of the unknown.