Why are boats referred to as female?
WAG- They are expensive to keep, yet fun to be around. 
On a more serious note, the figurehead on sailing ships might have some role. Superstition held that a female baring her breasts to the sea would calm rough waters.
Russian sea vessels are referred to as male.
::d&r::
Because they have smooth bottoms.
Eh, I got nothin’.
During WWII, the Kriegsmarine referred to its ships as female - except for the Bismarck, which was male.
My WAG:
Historically, boat people were virtually all male (sailors, military, fishermen, pirates, ocean liner crews, etc.). They lived a good part of their lives on the water, and had to deal with many potentially life-threatening catastrophes. The best metaphor they could come up with was their relationships with women, who were often thought of as unpredictable and stormy.