Sharks have been featured twice in Calvin and Hobbes.
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.
A shark appeared once as a minor alter ego of Calvin's. In its only appearance, it breached from the surface of the bathtub to attack a creature in distress, analogous to Calvin's mother.