He doesn't know how to let go and trust his son with what he wants to do. To survive he needs to trust others and accept himself. Right at the beginning of the film you can see how Marlin, as a father, really wants to make sure that Nemo is safe from any danger in the ocean. Nemo accidentally gets trapped in some kind of underwater plant. Marlin freaks out and rushes to get Nemo to safety. As soon as Nemo frees himself, Marlin questions him, asking if everything is okay. “Nemo, don't move! Don't move! You'll never get out of there alone. I will." She asks him questions like "Are you dizzy? Any fluid flowing?" Nemo simply answers "No" to every question because he is annoyed by how overprotective his father is. In another scene, Nemo is going on a trip with his school. But Marlin doesn't know that they are going to the delivery point. He has trust issues with anyone and anything. As soon as he finds out, he rushes to find his son who scolds him once again for trying to swim in the open sea. Then it seems that Nemo has had enough of his father. When no one is looking, Nemo swims towards the boat to touch it. Marlin has protected his son for too long and it seems he has created a rebel within himself