Hi, This is Sei.
I am going to show my opinion what Ponyo means in reality. Ponyo is the symbol of environment and Sosuke represent human being. At first, Ponyo wanted to be a human because she fell in love with Sosuke. It means nature is impacted by human. Actually, when Ponyo changed as a human and she was finding Sosuke, there was a tsunamai. Because at that time balance of nature was broken by Ponyo.
So human have an bad influence on nature. However, Sosuke also love Ponyo, not only as a human but as a fish. it means that human truely love environment. And Sosuke and Ponyo is young. It means eventhough there was big flood becasue of tsunami, the future is positive when we love our environment like Sosuke. Also Sosuke's mother and Ponyo's mother were talking, it means human commune with nature. After that, Sosuke overcome the difficulty. Therefore, Ponyo tell[s] us if we truely love, commune with nature and keep balance, our future will be bright.
I am touched by the director because his message was delivered to me as a beautiful fairy tale with an image beauty. Eventhough there are alot of films which are talking about environmental problem, this point makes Ponyo special.