Responsa for Yevamot 241:9

Most scholars hold the opinion that a woman whose husband fell into a body of water whose final boundaries are not discernible, and who, though not permitted to remarry, defiantly transgressed against the prohibition of the Rabbis and did remarry, need not be divorced from her second husband. This opinion they base on the talmudic ruling that the law prohibiting the remarriage of a woman whose husband fell into a "boundless" body of water, applies only before, but not after, she has remarried. I once saw a prominent personage of our kingdom remarry under such circumstances without any one admonishing her. All our prominent scholars decided that she need not be divorced, once she has remarried. A great French scholar was then staying in our kingdom, and he, too, permitted her to stay married claiming that he had witnessed a similar case in France and that all the great scholars of France ruled that she need not be divorced. I also gave my consent to his decision though I was hesitant. However, I have finally come to the conclusion that such an opinion is untenable. For, were this opinion correct the prohibition of the Rabbis would be rendered senseless, useless, and ridiculous, since no woman would heed such prohibition and would transgress against it rather than stay single all her life. Why, then, would the Rabbis prohibit such remarriage to begin with! Therefore, we must say that the talmudic ruling which does not require the remarried woman to be divorced, refers only to that woman who remarried because she innocently believed that a scholar had permitted her to do so, as in the case of Hassa (Yeb. 121b) and that of R. Shila (Yeb. 121a); but that it does not refer to the woman who defiantly transgressed the prohibition of the Rabbis. This latter interpretation of the pertinent talmudic sources clarifies the words of the Rabbis and renders them logical and tenable. Therefore, a woman who remarried without receiving permission from a scholar must be divorced, or the husband be put under the ban till he divorce her. We must not be concerned lest a relative of the woman permit her to remarry (thus flouting the prohibition of the Rabbis), for the following reasons: a) only a scholar holding the position among scholars that R. Nahman (the scholar who inadvertantly caused the wife of Hassa to remarry) and R. Shila held in their day, is empowered to permit such a woman to remarry; b) a scholar permitting such a woman to remarry would bring upon himself the ban.
SOURCES: Cr. 194; Am II, 97; Mord. Yeb. 128; Tesh. Maim. to Nashim, 11, Pr. 612; Tashbetz, 468. Cf. Weil, Responsa 128.