The real reason why boats are referred to as 'she' in English is unknown.

But there are a number of possible explanations-- most seem to be of the folk-tale variety, invented after the fact as a way to explain the phenomenon. Boats are a truly interesting case in English, as they are among the only non-living objects that take a gendered (male/female) pronoun, whereas virtually all others are named with the neutral it.

A theory that is plausible is that boats are called she because they named after a significant woman in the boat-owner's life--a girlfriend, wife or mother.

It has also been suspected that ancient ships were once consecrated to goddesses and from that it slowly evolved to naming after any woman. It is interesting to note that, although male sailors historically attributed the spirit of a benificent female person to their ships, actual women were thought to be very bad luck when at sea.