Ishmael is the objective voice of reason and insight that comes to understand and interpret Ahab's monomania. Ishmael even goes so far as to garner some of the same feeling as Ahab himself possesses. In other words, he is willing to see the White Whale as the emblem of that element of things unseen that is horrible:
“Though in many of its aspects this visible world seems formed in love,” he says, “the invisible spheres were formed in fright.”
Ishmael's understanding and acceptance of Ahab's stance also allows him to be the philosophical voice that realizes that multiplicity paradoxically embodies unity while diverse parts paradoxically generates oneness. Specifically, the White Whale has no one definitive meaning, only a multiplicity of meanings that ultimately unite while the diverseness of individuals on the Pequod generates oneness as each contribute to the whole of experience. In addition, as suggested in the quote, Ishmael understands the potential for evil in the "terrible unseen."
Retribution, swift vengeance, eternal malice were in his whole aspect
It is for these reasons, his role as empathizer with Ahab and his role as insightful philosopher carrying Melville's message, that Ishmael is the one to survive and is the sole survivor. n other words, the messages of unity in diversity, of oneness in diverse groups, of potential evil in the unseen carry the importance of the novel; overriding importance is not given to the fate of the characters.
One important, if practical, reason for Ishmael's survival is that it is through him that we know anything about Ahab and the hunt for Moby Dick at all—no Ishmael, no story. Ishmael functions as character, narrator, plot device. To the extent that he serves as the reader's window into Ahab's worldview, he also comes to function as a sort of reader himself—that is, as readers, we empathize with him somewhat, and his experiences become ours. In that sense, his survival is also our survival.
I think it is a mistake to try to understand Ishmael as an ordinary character in a book. While it is true that early on, during his meeting with Queequeg at the Spouter Inn, for instance, he can be seen as a comic figure, he also functions as a voice of judgment, both of Ahab and of the reader (as when he challenges the reader at the end of the chapter "The Whiteness of the Whale"—"Wonder ye then at the fiery hunt?"). At other times he is a philosopher, commenting on the nature of reality, as in "The Mast Head," or of desire ("A Squeeze of the Hand"). Sometimes he drops into the voice of a standard narrator, as in "The Quarter Deck"; other times, he disappears altogether (think of the parts where the novel becomes a play, as in Chapter 40, "Midnight, Forecastle"). So his survival is not just the lucky chance of finding a floating coffin to cling to after the destruction of the Pequod—it also has to do with stepping back from an abyss of self-negation, beyond which no book (or existence, even) is possible. It is like waking from a dream. As Ishmael puts it in "The Masthead," "But while this sleep, this dream is on ye, move your foot or hand an inch; slip your hold at all; and your identity comes back in horror. Over Descartian vortices you hover."