Darlin' article from Wikipedia: http://en.wikipedia.org/wiki/Ship's_cat. Basic history, plus sweet and noble tales of real ships' cats, fictional usages, and loads of good links.
p.s. : I hope all you lovers of our furry friends know about the mystery-solving animals in the (series) works of Shirley Rousseau Murphy and the dynamic duo of Rita Mae Brown and Sneakie Pie Brown. They're my top favorites, but web searching ("Cat mysteries" for instance) will lead you to many more, including those in which animals are significant but peripheral...But can be excellent, nonetheless, such as Blaze Clement's topnotch amazing combination of 'hardboiled' and heartfelt sentiment in her series starring Dixie Hemingway, Siesta Key's ex-deputy sherriff petsitter.