"Thank you very much," Orr said, and shook hands with his boss. The big green flipper was cool on his human hand. He went out with Heather into the warm, rainy afternoon of summer. The Alien watched them from within the glass-fronted shop, as a sea creature might watch from an aquarium, seeing them pass and disappear into the mist. (11.59)
The novel ends the same way it began, with more sea imagery. In fact, the whole story has been kind of fishy. We began with the jellyfish floating along in the sea that turned into dry land, then we figured out that the jellyfish might have been George, and then, after a particularly weird dream, we get aliens that just happen to look like giant sea turtles, flippers and all. Last but not least, these aliens keep talking about "crossings in the mist."
All of that comes together in this last paragraph. While the jellyfish in the beginning was wrenched out of its natural environment, George and Heather find themselves surrounded by sea like imagery, as if it has become their home. As they disappear into the mist, it's almost as if they are returning to their natural place in the ocean. Order has been restored, the world has returned to nature, and everything is going to be okay.