Finding Nemo has a pretty simple narrative structure. We start with a flashback from when Nemo was just a little egg and then jump ahead to his first day of school. From then on out, things pretty much happen one right after another. Marlin panics. Nemo gets captured. Marlin swims after him.
Where the movie does change things up a little is that it starts to tell both Marlin and Nemo's individual stories once they're separated. We get to see Marlin hunting for his son and Nemo adjusting to life in the fish tank at the same time. This is effective for two reasons.
First, it lets the audience know exactly what's going on with both our main characters. We're not left wondering if Nemo's been served up on a fish sandwich as Marlin makes this dangerous trip across the ocean. We know that Nemo is alive and well and that Marlin just has to keep swimming to get to him.
It's also effective because the split narrative is giving us—the audience—information that the characters don't have, thus creating super dramatic moments. This works especially well during the climactic scene in the dentist's office. Nemo's playing dead to escape just as Marlin has arrived. We know that Nemo's faking, but his dad doesn't. We understand the stakes in this scene, but Marlin and Nemo don't have the full picture yet.
Even though it all turns out okay, we can't help but feeling that a simple misunderstanding is going to keep father and son from getting back together—a testament to the power of multiple points of view.