Blue Diving Suit
Collect all nineteen shells and you will then be able to obtain the blue diving suit. To do so, first exit the spirit world. Then follow the paths around your shell collection and interact with the murals and nearby walls. With the suit equipped, you can swim much more quickly.
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.
