Is there a law that if both your son and daughter are drowning, and you have the ability to save either one but only time enough to save one of them, you are obligated to save the son?
Yes there is such a law, and hopefully it will never be practical. The reason for this is not because men are better than women, but because men have more mitzvos than women do. The continuation of the mishnah states that if there is only enough clothing for one person then the clothing should be given to the woman, and if the two of them are being held prisoners that the woman gets ransomed and redeemed before the man. The reason for this is since she is more embarrassed and in more danger than the man.
Tractate Horios 3-7