Blue Suit
After you collect all nineteen shells, you can equip and use the blue suit which makes you swim faster. To use it, after coming out of the spirit world, fallow the path into the walls with the murals, interact with the wall inside with the shell collection.
Finding Nemo Easter Egg
You can find a clown fish and a blue tang fish playing together, a reference to Pixar's animated films about Nemo and Dory. To do so, make your way through the Whale Shark level until you go through the first major gate, where you'll activate two fountains. Look for a hole in the ceiling, which you can swim through to find the two lone fish.