A very Freudian kind of story. The "water of the womb" and the "desire to return" are two theories of his which are borne out in this story in extremely literal terms. I like the author's take on it: that the desire to return to the womb is ultimately a fantasy, and a dangerous one at that. Succumb to it and you end up just being food for the fishies. That isn't to say that there are good reasons for escapism: we all saw the office lady's complaints, being overworked and sexually harassed... Compare and contrast with the clinical trial story, where we see a similar perception of reality: sour, but ultimately better than a death-dream. Actually, I should write a post one of these issues about the persistent theme of dreams and how the manga uses them.
I know this is Japanese, but this perception of life reminds me somewhat of Confucianism: life is sour. Not bitter like in Buddhism, nor sweet like in Daoism. Life is sour: flawed, and a little rotten, but ultimately you have to eat it.