When trying to establish a theme of a particular work, I think that some questions have to be asked. The first would be what you think the message of the work is. What would this be in relation to "Finding Nemo?" You could take it directly: Nemo learns to understand that belief in oneself can overcome almost any condition. Marlon, his father, learns to trust his son, and not capitulate to his fears. Certainly, the belief in self and the trust in others could be themes that emerge from the film. Another theme could lie in the strength of community. The animal kingdom works together in helping Marlon reunite with Nemo. Think of the many examples of this in the film and you have another theme.
First, what a great movie! Second, there are many themes. So, I am sure that people will have different answers. Before I give it a shot, let me say one thing about the name "Nemo." In Latin the name means "no one." I wonder if this has something to do with the theme. I am not sure, but it is something to think about.
I think the major theme is a father's love for his son. Father's may not know how to love perfectly and they might make many mistakes long the way, but a father will do anything for his son, even die. I think Marion shows this as he risks it all to find Nemo.
Frequently, the title of a work points to its theme or the author's main idea. The title Finding Nemo definitely gives viewers a clue about the theme as well as a hint at the complexity of what might be seen as a simple children's story. While the story is about a father's search for his son, it's also about the son's journey towards manhood. Both have to overcome substantial obstacles and, in doing so, both achieve their goals. Marlon finds his son, but it is a very different son from the one at the beginning of the movie. Nemo finds strength of character beyond his greatest expectations. The love the two share is stronger than ever, but now there is also mutual respect. A thematic statement could reflect how the power of love can lead to self-discovery.