Okay, we know why you're here. There you were, just reading away, and then: bam. Shark. You've just got to know what's up with that carnivorous fish, right?
We've got plenty of ideas about that, but you're going to have wait for us to discuss the rest of the ending first. Or, if you're only here for the sharks and nothing but the sharks, just scroll down to our shark section.
So we've come to the novel's end. It's a small chapter, but one jammed with important imagery.
The Quo Peregrinatur is boarded by children and monks, and nuclear weapons are blowing the world back to something that would make the Stone Age look advanced by comparison. Then:
The last monk, upon entering, paused in the lock. He stood in the open hatchway and took of his sandals. "Sic transit mundus," he murmured, looking back at the glow. He slapped the soles of his sandals together, beating the dirt out of them. (30.5)
Finally, the ship launches "itself heavenward" with the Memorabilia (30.6).
For starters, let us note that this is one of the few places where children are specifically mentioned in the novel. Most of the story takes place at an abbey, so we haven't seen a lot of kids running amok before.
The only other notable mention of children in the whole novel is the poor child who suffered from radiation sickness. In contrast to that kid, these kids have survived the nuclear holocaust, and so they symbolize what you'd expect children to symbolize: the future of humanity.
Then we come to the monk. His murmuring translates to "so goes the world" or "so passes the world" and his removing the dirt from his sandals evoke his act of leaving the Earth (literally and figuratively) behind (source). He doesn't want to take the Earth with him as he ascends into space. It's a fresh start. With clean feet.
Calling the move into space a "heavenward" journey hints that better times awaits those who have managed to leave the fiery hell of Earth. In fact, in many classic science fiction stories, the move to space results in transcendence for humanity as it shows us leaving our primitive, old school problems behind for something better.
You can check out Arthur C. Clarke's Childhood's End for a classic example. But the Memorabilia remain.
As Joshua notes earlier, the Memorabilia could be a blessing or a curse. That depends on whether or not the Memorabilia are "perverted by Man, as fire had been" (26.60). So, we're left to wonder what will become of humanity as it colonizes the stars.
Space travel might symbolize a new beginning, but it's no guarantee. Will the cycle finally be broken when humanity moves into space, or will it merely repeat itself on a cosmic scale? Will our hopes for humanity's progress amount to nothing?
Now onto that confounding shark.
After the world has been laid to nuclear waste, the ocean washes driftwood and a plane ashore. Then the dead shrimp and dead fish flow onto the beach. And at long last, a "shark swam out to his deepest waters and brooded in the old clean currents" (30.8). He's "very hungry that season" (30.8).
Let's not forget that these beasties have been around a long time. Approximately 200 million years before the earliest known dinosaurs, to be exact (source). Sharks were built to last.
So the shark returning to its "old clean currents" suggests that it will survive the fallout, even if its survival will be a hungry one. Life on Earth will not vanish, but simply regress back to an older state—the one that existed well before us people-things arrived.