What is the symbolism of the ocean in the novel, After Dark?

The ocean is a symbol of loneliness and isolation. Eri is frequently described as being at the bottom of the ocean while she sleeps. Mari and Shirakawa both watch a television program about deep sea creatures. Shirakawa watches the whole program, but Mari shuts it off to speak with Komugi. The two open up to one another, fighting the isolation away.