The ship is a she because the sailors of old personified them. They worked everyday on this ship, they depended on it, and they had a relationship with it. They knew the ship as you know a person with its own quirks and teperatment. Traditionally, this persona was that of a woman. Perhaps it had something to do with the way the men took care of her, as men cared for women. Perhaps they just didn't have any real women around at sea, so they needed the ship to be female. There is a lot of traditional lore about the special relationship that the captain had with the ship: she was his mistress. At least, that is what I've heard.