The sea monster Cetus is a symbol of the wrath of the gods, particularly the wrath of Poseidon, the sea god. When the mortals misbehave, Poseidon sends Cetus after them.
Over the years, the Cetus became a symbol of all the bad stuff that can happen at sea – storms, pirate attacks, shipwrecks, stuff like that. Superstitious sailors wouldn't even say the name Cetus for fear that bad things would happen. On the other hand, some ships would fly in the face of superstition and name their ship Cetus, showing that they were unafraid of the dangers of the sea.