The main character of this story is the boy.
He wanted people around him to know about happy time with mermaid, but they had abandoned their ability of imagination because of the national power, so they didn't believe what he said.
National power tried to deprive him of his ability of imagination, and he barely lost it, but finally, he could meet mermaid again.
I think that Tezuka Osamu wanted to assert that everyone has a right to imagine as she or he likes.
