Even in the absence of a moral obligation, saving a drowning child to whom you are not otherwise connected (or getting first aid training in anticipation of such an opportunity, etc.) might still be a very worthwhile investment, with the right follow-up. In addition to broader reputational effects, there's the possibility of a debt of gratitude from the child and any associated parents or guardians which would broaden and diversify your social circle, without giving those approached an opportunity to resent the intrusion.