Symbolism, Imagery, Allegory
Not to oversimplify things here, but the ocean and outer space represent the unknown in Childhood's End. The only difference is which direction you have to travel to get there.
Jan Rodricks starts his journey in search of the unknown by going into the ocean. His dark, deep descent into the undersea canyons is full of mysteries and things barely understandable lurking in the distance. In fact, Jan compares the deep-sea diver vessel he travels in to a "spaceship" (11.25) and a luminous deep-sea fish is described as swimming "like a meteor into the darkness of the abyss" (11.43). So yeah—the ocean and space are pretty interchangeable here.
Later in the novel, we learn Jeff Greggson is "fascinated" (16.1) with the ocean and exploring the mysteries of the lagoons and beaches surrounding the island of Sparta. Given Rodricks' comparisons of the sea to space, it's no coincidence that the child who loves to play in the ocean is also the child who becomes the catalyst that leads mankind into space as part of the Overmind.
So, whether you go below sea level or well above it, Clarke is using images from each to represent the great and unknown.