This is simply a manner of etiquette. These creatures are never taken by sailors from other ships, because it is very bad luck to steal another ship's marine cat. On the vessel a marine cat can get into any area. Sailors often find their cats up in the rigging, in locked staterooms, or sleeping in the weapons magazines. A marine cat loves to generate surprise and will seek to position itself high enough that when a nearby person turns around he is staring right into the cat's unblinking eyes.
Sailors often feed their cats by hand, offering pieces of fish and beef from their own plates.
Marine cats also hunt the cargo holds, feeding on rats and keeping the ship free of voats. Strangers taking passage on the vessel will find themselves under constant scrutiny by the cat, who likes change only when it is the one causing it. Marine cats, lucky or not, do seem to protect the welfare of the ship. If someone is not where he is supposed to be, likely as not he will step on the cat's tail, causing it to cry out and notifying everyone nearby of his presence. When a lookout falls asleep, oblivious to a nearby danger, the cat may then decide that the man's earring makes a perfect toy.