You're assuming that a woman wouldn't have a reason or desire to go to sea for the same reasons as a man would, particularly since the laws at the time made it difficult for her to earn a decent living on her own. Although I agree that the difficulties of disguising one's sex in such close-quarters as a ship would discourage most women, I'm guessing that most of the women who served went undiscovered. Or, how many of these women could have been thrown overboard by a superstitious crew (women being supposedly bad luck on ships and all) and their deaths recorded as accidents or casualties of war (and listed with their male name so as to avoid the stigma of bad luck)? Or have been killed in a melee or storm and their body washed overboard before they could be examined and discovered as women? At any rate, I think it's a safe bet that as with most things, the women who were documented were just the tip of the iceberg.