it used to be pretty common for there to be cats on boats. they'd take care of any rodents who would chew on ropes or wires and spread diseases. sailors were also superstitious and believed that having a cat aboard would bring good luck! this belief passed on to their wives, who kept cats - especially black ones who were believed to be extra lucky - at home in order to keep their husbands safe when at sea.
another popular superstition? that if a cat came aboard it was a sign of luck, but if it only boarded halfway and then left, it was a sign of bad luck.
most ship's cats are only found in modern times on private vessels, but they have roots going back to early history. one such example is the Vikings, who took cats with them on expeditions.
Original Alex Toth art from Dale Evans Comics #9 (DC, 1950). Alex Toth turned in a cool mystery in the Sierra Smith, Western Detective series, “The Case of the Haunted Horse!” Produced in ink over graphite on Bristol board with an image area of 13" x 18". Panel 1 and upper left corner has ink from brush cleaning.