Yevamot 121 - A man fell into water
If one fell into a water reservoir, whether a small one, such that all of it can be observed, or one larger than that - he might still be saved, and his wife cannot re-marry based on this testimony alone. Rabbi Meir quotes an incident to support this ruling: there was a man who fell into a large water cistern, and emerged after three days. This proves that even in such situations there is a possibility that the construction of a cistern or a pond will allow him to survive. Certainly in a large body of water he could have been saved by waves, for example, and emerged for away, out of the sight of observers.
However, Rabbi Yose and other Sages disagree. Rabbi Yose quotes a different incident, and even a ruling: one blind person went into a cave, to immerse himself for ritual purity, but fell in. His assistant descended after him and also fell in. After the time period that it would take for them to surely die, the court allowed their wives to re-marry, even though the bodies were never recovered.
What do the Sages answer to Rabbi Meir's incident? - They say that it was a miracle anyway, and it cannot serve a basis for legal rulings. Why do they call it a miracle? If it is because he did not eat for three days - why, Ester and the whole Jewish people fasted for three days and survived. Rather, it is because he did not sleep for three days, and that is impossible. And Rabbi Meir? - He leaned upon the arches that were inside the cistern and slept. And the Sages? - The arches were of marble, and he would have slipped and fall. And Rabbi Meir? - He would wake up before falling. Thus, each can defend his point of view.
Art: Found Drowned by George Frederick Watts