Complex sentence: Superstitious sailors believed in a "cetus" as the bringer of a great storm or misfortune to the ship, that is lost cargo, pirates, or being swept off course, and avoided any talk of it aboard ship.
Split into simpler sentences: Superstitious sailors believed in a "cetus" as the bringer of a great storm or misfortune on the ship. They associated it with lost cargo, the presence of pirates, or being swept off course, and avoided any talk of it aboard ship.