Rescuing the two sisters from drowning can not be a reason for considering the young man as a Mahram to them. They are not permitted to take off their Hijab in his presence as he is a non-Mahram. It is permissible for him to marry one of them as long as there is no impediment, such as breastfeeding that causes prohibition. May Allah grant us success. May peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions.