Responsa for Yevamot 241:21

תנו רבנן נפל לגוב אריות אין מעידין עליו לחפורה מלאה נחשים ועקרבים מעידין עליו רבי יהודה בן בתירא אומר אף לחפורה מלאה נחשים ועקרבים אין מעידין עליו חיישינן

then also in the case of water which has [a visible] end the possibility of having remained in a subterranean fish pond should be taken into consideration! — It is not usual for a subterranean fish pond to be found with water which has [a visible] end.<span class="x" onmousemove="('comment',' [There is not sufficient fish to warrant the construction of a pond (Me'iri)]. ');">³⁸</span> R. Ashi said: The ruling of the Rabbis [that where a man has fallen into] water which has no [visible] end his wife is forbidden [to marry again]. applies only to an ordinary person but not to a learned man for, should he be rescued.<span class="x" onmousemove="('comment',' Lit., 'that he went up' ');">³⁹</span> the fact would become known.<span class="x" onmousemove="('comment',' Lit., 'he has a voice'. ');">⁴⁰</span> This, however, is not correct; for there is no difference between an ordinary man and a learned man. <i>Ex post facto</i>, the marriage<span class="x" onmousemove="('comment',' Of his wife to another man. ');">⁴¹</span> is valid; <i>ab initio</i>, it is forbidden. It was taught: R. Gamaliel related, 'I was once travelling on board a ship when I observed a shipwreck and was sorely grieved for [the apparent loss of] a scholar<span class="x" onmousemove="('comment',' Talmud Hakam, v. Glos. ');">⁴²</span> who had been travelling on board that ship. (And who was he? — R. Akiba.) When I subsequently landed, he<span class="x" onmousemove="('comment',' R. Akiba. ');">⁴³</span> came to me and sat down and discussed matters of halachah. "My son", I asked him, "who rescued you?" "The plank of a ship", he answered me, "came my way, and to every wave that approached me I bent my head" — 44 Hence the Sages said that if wicked persons attack a man let him bend his head to them.<span class="x" onmousemove="('comment',' Cf. supra n. 6 ');">⁴⁵</span> At that hour I exclaimed: How significant are the words of the Stages who ruled [that if a man fell into] water which has [a visible] end, [his wife] is permitted [to marry again; but if into] water which has no [visible] end, she is forbidden'. It was taught: R. Akiba related, 'l was once travelling on board a ship when I observed a ship in distress,<span class="x" onmousemove="('comment',' Lit., 'that was tossed in the sea'. ');">⁴⁶</span> and was much grieved on account of a scholar who was on it. (And who was it? — R. Meir.) When I subsequently landed in the province of Cappadocia<span class="x" onmousemove="('comment',' [G] in Asia Minor. ');">⁴⁷</span> he came to me and sat down and discussed matters of halachah. "My son", I said to him, "who rescued you?" — "One wave" he answered me, "tossed me to another, and the other to yet another until [the sea] cast me<span class="x" onmousemove="('comment',' Lit., 'vomited me out'. ');">⁴⁸</span> on the dry land". At that hour I exclaimed: How significant are the words of the Sages who ruled [that if a man fell into] water which has [a visible] end, [his wife] is permitted [to marry again; but if into] water which has no [visible] end, she is forbidden'. Our Rabbis taught: If a man fell into a lion's den, no evidence<span class="x" onmousemove="('comment',' That he is dead. ');">⁴⁹</span> may be legally tendered concerning him;<span class="x" onmousemove="('comment',' To enable his wife to marry again. ');">⁵⁰</span> but if into a pit full of serpents and scorpions, evidence<span class="x" onmousemove="('comment',' That he is dead. ');">⁴⁹</span> may legally be tendered concerning him.<span class="x" onmousemove="('comment',' To enable his wife to marry again. ');">⁵⁰</span> R. Judah b. Bathyra ruled: Even [if he fell] into a pit full of serpents and scorpions, no evidence<span class="x" onmousemove="('comment',' That he is dead. ');">⁴⁹</span> may legally be tendered concerning him,<span class="x" onmousemove="('comment',' To enable his wife to marry again. ');">⁵⁰</span> since the possibility must be taken into consideration

Teshuvot Maharam