I am not knocking any of the other suggestions, but adding to the list of theories.
It would be a shame to knock a post dead after a couple of posts.

My own theory is as follows (and like others, may be shot down as a load of tosh):

Traditionally, men designed, constructed, manufactured, assembled, used and serviced ships, cars, aircraft, buses, lorries etc.
The industry was a masculine domain and they were the days when allegedly, men were men.

An inanimate object, made from wood or metal being tended and cared for by a man, possibly needed to be categorised as you wouldn't have a man, tending and caring for another man...
Giving the mode of transport a 'female' name or referring to it as a 'she', could have made the caring and looking after it acceptable.
The crew of a ship going out to see, were proud of their vessel and wanted to show it off to their fellow sailors in the fleet and also the enemy.

It could be a load of tosh, but hey ho.