Here is a post from a similar question when asked to R’ Tendler.
Thank you for a very interesting question. First of all, I was unaware that such a Psak had been rendered in a public manner, so I really can’t help you in determining which Rav may have announced this Psak. Most religious women that I know will only swim at a pool where there are female lifeguards, or will wear a robe if there are males present. I am aware, though, that at separate beaches in Israel it is common for women to swim in the presence of male lifeguards, the rationale being that most women lifeguards may not be strong enough to carry out an ocean rescue if CH”V needed. However, even in this situation, many women will only go into the ocean in their robes, and this is certainly preferable, when possible.
However, you are correct in observing that some religious women don’t have a problem swimming in the presence of male lifeguards in their bathing suits. In their defense, I’d like to explain as follows. The regular Chiyuv to dress in a modest manner only applies in a public area, where Bnos Yisrael dress in this fashion. At a swimming pool or on the beach, if it is designated for women only, it would be permitted to dress as Bnos Yisroel normally do in this venue, in their bathing suits. However, there is another problem, that of “Lifnei Iver”, appearing in front of a man in a manner in which he will have improper thoughts, which applies in all situations. In Halacha we find a concept of “Torud B’Melachto”- when a man is involved in doing a job, we say that he is busy concentrating on properly doing his job, and you are not transgressing Lifnei Iver be appearing in front of him in an undressed manner since he will not be easily distracted from his job, as he would be if he just happened to be sitting there. This can be compared to a male doctor who is doing a routine checkup on a female patient, he is busy doing his job in the most professional manner, and we don’t have to be concerned that his mind might wander. However, if you are certain that this is not the case, it would certainly not be appropriate to swim in the presence of such a lifeguard, or to allow yourself to be examined by such a physician.
I’m not certain what you mean regarding the difference between swimming and other sports. If you don’t mind explaining, perhaps I could better help you.
I hope that this is helpful for you.
Take care,
Rabbi Aaron Tendler