Q: What do a swimming instructor and a mohel have in common?
A: Both perform a mitzvah for children, in lieu of the children's own parents.
A mohel, of course, performs a brit mila (ritual circumcision), a mitzvah the Torah has commanded the father* of a baby boy, (and since most Jewish fathers are not trained to carry out on their own, they graciously pass the duty onwards, to somebody who is).
A swimming instructor teaches children to be water-safe, a mitzvah the Rabbis, via the Talmud, have assigned to the parents of all children, as an extension of the mitzvah to keep oneself alive. (Again, while most parents may be able swim, teaching is an entirely different matter and requires an expert. Go RivkA!).
Which means that both men and women can fill in for a child's parents on this one. In fact, when I think about it, this might be the only mitzvah a non-Jewish person can do on behalf of a Jewish person. What a cool thought.
(I know, I know... technically, the parent of the child is doing the mitzvah by paying for the lesson. Or in our case, the grandparents of our children. Thank you...)
Keep the balance,
*Only fathers are commanded to give their sons a brit mila since, generally, one is only commanded to perform a mitzvah for others if he himself is obligated to do it.