False shark
One way you know people that say that they are sharks are not sharks is sharks don't try to run off other animals. Sharks can also swim without the use of tanks and scuba diving apparatus, that's another way you know that people aren't like sharks, even if they claim to be.
He/she must have been in a silly mood when he/she claimed to be a shark, but nobody thought he/she wasn't a false shark when he/she made the claim.
False shark
One way you know someone claiming to be a shark is not a shark is sharks don't run other animals off. Another way you know someone who claims to be a shark is not a shark is sharks don't need tanks and scuba apparatus to swim deep underwater, their bodies are made for their environment, which is not a natural environment for humans.
The false shark said he/she was a shark, despite a lack of similarity to the underwater animals.