Because a ship carries, protects, and eventually delivers her cargo or passengers as does a mother with her child.
And because most boats up until at least 1970 were owned by men ::)
It's from Wikipedia, that means it has to be true,.... ;)
Or was it, she's out of control without a strong man at the helm,
shows her top, but hides her bottom,
always has a bunch of men tending to her
and when back in harbor, she heads straight for the buoys....
Yes dear, the corner does need vacuuming again, I'll get right on it..... :P