Let me just go ahead and spoil something for you. You do find Nemo on this ride.
The Finding Nemo ride took over the Submarine Voyage which was around until 1998. Nemo and his dad Marlin took over in June 2007. The ride still has parts of the of the original. You take off through a coral reef from the original ride. You spot Darla (the scary fish-killing niece) and soon you dive deeper in a fury of small bubbles. The entire gang from the movie starts showing up via projections on silk scrims. Marlin and Dory search fror Nemo throughout the ocean. Along the way they meet Crush, jellyfish, Bruce the Shark and his friends, scary fish with sharp teeth, Nemo, and a humpback whale. The submarine is in danger several times. Don't worry, you get through okay.
After you escape a whale, you head towards port. The sub captain says, "We better get out of here before we see a sea serpent or mermaid." This refers to the prior attraction which featured four mermaids and a gigantic sea serpent. Keep your eyes peeled because right after he says this, you can see two coral formations - one shaped like the sea serpent's head and the other shaped like a swimming mermaid.
It's a really adorable ride. Super fun sculpts and the projections are flawless. It is definitely worth at least one ride!