Sharks have been featured twice in Calvin and Hobbes.
Shark Test
Calvin once admitted to believing that there may be sharks in his bathtub. In order to disabuse himself of this notion, he placed a rubber duck toy to test for the presence of sharks.
Calvin the Shark
Calvin the Shark is a minor alter ego of Calvin's. In his only appearance, he breached from the surface of the bathtub to attack a creature in distress.
See also
Snowmen