Oh yeah your right, on the early ships, like the really early explorations vessels that still used the wind move and the stars to navigate they'd bring a cat on board to keep the rat population on the ships to a minimum. I think it would have been phased out between World War I and II cause the new ships design was far less hospitable to rats and they'd invented rat poison so it wasn't really necessary but it's really up to the GM. Though I think we should remember that this is the cat of a stowaway, shouldn't we be more concerned with what happens to the stowaway when he's found out?