The question came before Hacham Ovadia Yosef concerning a couple that happened to be in a place without a Mikveh when it was time for her to immerse. As they were near the beach, the woman decided to immerse in the ocean, wearing a bathing suit. Does this qualify as a Halachically valid Tebila (immersion), or does the bathing suit constitute a Hasisa ("interruption") in between her body and the water, thus disqualifying the Tebila?
Hacham Ovadia ruled (as written in Taharat Habayit, vol. 3, p. 182; listen to audio recording for precise citation) that the tight bathing suits which are normally worn for swimming would indeed constitute a Hasisa, as they prevent direct contact between the body and the water. Therefore, a woman who swam in the ocean in lieu of immersing in the Mikveh is still considered a Nidda, and relations with her husband remain strictly forbidden.
Hacham Ovadia adds, however, that if a woman wears a loose-fitted garment, like a loose robe, which allows the water to directly access the entire body, then the immersion is valid. He notes that this is indeed the way immersion should be done in the case of a female convert. The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) rules that when a person converts to Judaism, he or she must immerse in the presence of a Bet Din. In the case of a female covert, it was necessary to avoid problems of modesty, and thus the woman would immerse until her neck while the Hachamim were in the next room. They would then come in, see the woman dunk her head in the water, and then leave. However, this method was appropriate only in ancient times, when the water of the Mikveh was generally murky and thus sufficed to avoid immodesty. Nowadays, however, Mikva’ot are well-kempt and the water is thus exceptionally clean, such that the Hachamim cannot enter the room of the Mikveh even when the woman is in the water. Therefore, the proper way for a Bet Din to witness a female convert’s immersion is for her to wear a loose-fitted robe which covers her body but still allows for direct contact with the water.
Likewise, in olden times, there were occasions when no proper Mikveh was available, and women had to immerse in lakes, oceans or other public areas. The practice under such circumstances was to perform the immersion while wearing loose-fitted garments.
Hacham Ovadia notes in this context a well-known responsum by Rav Moshe Feinstein (Russia-New York, 1895-1986), in his Iggerot Moshe (Eben Ha’ezer 4:23), which could be misinterpreted as validating an immersion done while wearing a bathing suit. The responsum deals with the status of a boy born to parents who were not observant and thus presumably did not observe the laws of family purity. The question was posed to Rav Moshe whether one may allow his daughter to marry such a boy, in light of the fact that he was conceived in the woman’s state of Nidda. Should this factor be considered if he is otherwise a desirable match – well-mannered, God-fearing and Torah observant? Rav Moshe ruled very clearly that the parents’ lack of observance should not affect the boy’s status as a worthy match, noting, first and foremost, that prospective marriage partners should be evaluated based on their current character and religious standing. A person’s parents’ level of observance should not diminish from his eligibility. Rav Moshe then adds that in all likelihood, the mother had, at some point, bathed in the ocean and was thus purified from her Nidda status. He writes that although she did not have in mind for her swimming at the beach to purify her, this is irrelevant, since Tebila is effective regardless of intent. And although she did not observe the "seven clean days," this waiting period is required only if she is actually a "Zaba," and it is very possible that she did not, in fact, have this status. Rav Moshe also notes that although she wore a bathing suit, this does not necessarily disqualify the Tebila. At first glance, it appears that Rav Moshe considered immersion with a bathing suit a valid Tebila. Hacham Ovadia clarifies, however, that Rav Moshe did not write this as a conclusive ruling. He mentioned this only as a possibility to be considered, but not as an actual ruling allowing immersion while wearing a bathing suit. God-forbid, Hacham Ovadia writes, should a person mistakenly infer from Rav Moshe’s responsum that a woman may immerse while wearing a bathing suit.
It should be noted that although Halacha allows immersion while wearing loose-fitted clothing, as mentioned, a woman should not perform Tebila in the ocean without consulting with a Rabbi, as there are numerous other issues to consider. To name just a few, swimming in a public area is never something that should be encouraged, the sand might constitute a Hasisa, and there is nobody to supervise her immersion to ensure it is done properly. Therefore, although in principle a woman may immerse with loose-fitted garments at the beach, she must consult with a competent Rabbi for guidance to avoid other potential problems. The laws of Nidda are especially severe because they are subject to the punishment of Karet, and therefore extreme care must be taken to avoid all potential Halachic questions.
Summary: In principle, a woman may immerse in the ocean with loose-fitted garments in lieu of immersion in a Mikveh, whereas immersing while wearing a bathing suit is invalid. Nevertheless, before a woman decides to immerse in the ocean instead of a Mikveh, she must consult with a Rabbi because of the numerous Halachic issues that are entailed.