SeaRider is a special submarine created to think and act like a real fish while her passengers enjoy the ride without operating her every move as she has artificial intelligence and a personality all her own. While the humans know that SeaRider is a submarine with the size and appearance of a regular fish, Nemo, Dory, Marlin, and the rest of their friends see the pink fish as a brand new friend named Sheila by Dory after reading her 'nametag' on the side of her body. Due to this, the fish and other marine mammals form a bond with the pink fish and thus the people inside the sub end up expereancing the whole adventure not as a person in a submarine but as a brand new friend in the Finding Nemo/Finding Dory universe.
Role in the attraction
When guests ride on the "Nemo & Friends SeaRider" attraction, they become shrunk due to the SeaRider being developed with Chiziminium, named after a Japanese pun for shrinking, just as the sub and the guests shrink upon boarding it. While boarding it, Dory approaches the sub and nicknames the fish sub "Sheila", thinking the vehicle the guests are boarding is a fish. Since the scene differs each time guests ride on the attraction, they are taken to a different scene via SeaRider.
Once shrunk, guests arrive either in a hide-and-seek scene or the scene with Mr. Ray (though in either way, guests are invited to join the fun) while Dory reads the SeaRider's name and calls her "Sheila", alongside Nemo, Marlin, and the others. When the SeaRider arrives at the drop off, the sub either follows Mr. Ray to watch Nemo and Dory or arrives at the scene where guests encounter Hank while Dory introduces Sheila to the others.
SeaRider then arrives at the East Australian Current where guests encounter Crush or Squirt and during the moment, Squirt hits Sheila so hard across the EAC before leaving. Either guests arrive at the Jellyfish forest where Marlin saves the sub before bouncing across the jellyfish or a kelp forest with a pack of Otters where they swim around the habitat with Nemo, Dory, Marlin, and Sheila playing tag with them.
Afterwards, guests arrive at a random climax scene either at the touch pool area or the underwater container area. For the touch pool, guests meet Hank outside of a pipe just as Nemo, Dory, Sheila, and Hank reunite while navigating followed by Darla (a character who made a non-physical cameo in the sequel) who grabs Sheila before realizing that she is not a real fish while Hank scares her away while inthe underwater container area, Marlin asks Dory to call Bailey and Destiny via whale emphasis before the Giant Squid emerges and attacks Dory's friends (including the SeaRider) while Bailey and Destiny save their friends from the squid.
Having escaped, guests are grabbed by Becky and a flock of loons carrying them passing some rocks and the sea lions Gerald and Fluke and Rudder or through a stingray migration while Marlin sings along. Arriving back home at the Great Barrier Reef, Sheila returns to normal size as everyone bids a farewell to the guests for a fun adventure across the ocean.