A woman who is a Nidda can divest herself of this status only by immersing in a Halachically valid Mikveh, which is defined as 40 Se'a of naturally collected rainwater, that had not been drawn from another source. This means that a Mikveh must be constructed in such a way that rainwater falls directly into the Mikveh. If the water is brought to the Mikveh from somewhere else, either through human involvement or a piping system, it is invalid, and a Nidda who immerses in such a Mikveh is still considered a Nidda.
For this reason, Chacham Ovadia Yosef rules (in Halichot Olam, vol. 5, p. 143) that a swimming pool may not be used as a Mikveh. The water in a swimming pool has the status of Mayim She'uvim – "drawn water" – and is therefore ineffective in ridding a woman of her Nidda status.
When Chacham Baruch Ben Chaim Z"L served as Rabbi in Salsbury, South Africa, he sent a letter to Chacham Ovadia Yosef asking if he could permit the women in his community to immerse in swimming pools, given that no proper Mikveh was available. He noted that if he would not permit women to immerse in swimming pools, they would resume relations with their husbands without immersing altogether. Since pools are disqualified for use as a Mikveh only Mi'de'rabbanan (by force of Rabbinic enactment), perhaps it is preferable to instruct women to violate this provision, and immerse in pools, so as to avoid their violation of the far more grievous transgression, of relations during a state of Nidda.
Chacham Ovadia replied that a Rabbi may not allow people to commit a Rabbinic prohibition even in such a case, where they will otherwise violate a Torah prohibition. Halacha does not become more flexible in response to the "intimidation" of those who are prepared to transgress Torah law if a Rabbinic provision is not suspended. This principle is explicitly mentioned as well by Rabbi Moshe Feinstein (Russia-New York, 1895-1986), in his work Iggerot Moshe (Yoreh Dei'a section, 52; listen to audio for direct citation), where he writes that a Rabbi may not allow a violation of Halacha of any kind due to the threats of sinners that they will otherwise commit more grievous transgressions.
Chacham Ovadia added another factor, as well, namely, that issuing a ruling permitting the use of pools as a Mikveh eliminates any incentive on the part of the community to construct a proper Mikveh. For this reason, too, a Rabbi must not issue such a ruling, and should rather insist that the community take it upon itself to construct a valid Mikveh.
Summary: A swimming pool may not be used in lieu of a Mikveh, even if no proper Mikveh is available.
