Shabbat 41A

דלית ליה גידודי, הא דאית ליה גידודי.

that does not have embankments surrounding it. Since there are no partitions, it appears like an ocean or a river. That incident involving R' Abbahu occurred in a place that has embankments and looks like a vessel. Therefore, the rabbis did not prohibit it.

ואמר רבי זירא: אנא חזיתיה לרבי אבהו שהניח ידיו כנגד פניו של מטה, ולא ידענא אי נגע אי לא נגע. פשיטא דלא נגע, דתניא רבי אליעזר אומר: כל האוחז באמה ומשתין ⁠— כאילו מביא מבול לעולם.

After citing what R' Zeira related with regard to R' Abbahu, the Talmud cites that R' Zeira said: I saw that R' Abbahu, while he was bathing, placed his hands over his genitals for the sake of modesty, and I do not know whether he touched them or did not touch them. The Talmud questions R' Zeira’s uncertainty. It is obvious that he did not touch his genitals, as it was taught in a baraita: R' Eliezer says: One who holds his penis and urinates it is as if he were bringing a flood to the world. He is liable to become aroused by that contact and that is an extremely severe transgression, comparable to the transgressions violated in the Generation of the Flood.

אמר אביי: עשאוה כבולשת. דתנן: בולשת שנכנסה לעיר, בשעת שלום ⁠— חביות פתוחות אסורות, סתומות מותרות. בשעת מלחמה ⁠— אלו ואלו מותרות, לפי שאין פנאי לנסך. אלמא כיון דבעיתי לא מנסכי. הכא נמי, כיון דבעית לא אתי להרהורי. הכא מאי ביעתותא? ביעתותא דנהרא.

Abaye said: Nevertheless, no proof can be cited from that baraita. Perhaps the rabbis rendered the legal status of this situation like that of a military unit, as we learned in a Mishnah: A military unit that entered a city, if it entered during peacetime, after the soldiers leave, the open jugs of wine are prohibited and the wine in them may not be drunk due to suspicion that the non-Jew soldiers may have poured this wine as a libation for idolatry. The sealed jugs are permitted. However, if the unit entered in wartime, both are permitted because in wartime there is no respite to pour wine for idolatry, and one can be certain that the soldiers did not do so. Apparently, since they are afraid, they do not pour libations. Here too, in the case of bathing, since he is afraid, he will not come to have impure thoughts. The Talmud asks: And what fear is there here that would prevent one bathing from having impure thoughts? The Talmud answers: Fear of the river. Since he needs to be careful that the water does not wash him away, he is too distracted to think of other matters.