Eruvin 43A

להבריח מים עשויות.

for they are only made to keep the water out; that is to say, a boat’s walls are not designed to turn it into a place of residence, but to protect it from the water. Therefore, they do not have the status of partitions made for the purpose of residence.

ורבה, מאי טעמא לא אמר כרבי זירא? במהלכת ⁠— כולי עלמא לא פליגי. כי פליגי בשעמדה.

The Talmud asks: As for Rabba, what is the reason he did not state his opinion in accordance with the opinion of R' Zeira? The Talmud answers: With regard to a boat that is moving, all agree, i.e., even R' Yehoshua and R' Akiva, that one is permitted to walk about the entire boat. They disagree only with regard to a boat that is stationary. Rabban Gamliel holds that the boat’s walls constitute effective partitions, whereas R' Yehoshua disagrees.

אמר רב נחמן בר יצחק: מתניתין נמי דיקא דבמהלכת לא פליגי. ממאי? מדקתני: מעשה שבאו מפלנדרסין והפליגה ספינתם בים. רבן גמליאל ורבי אלעזר בן עזריה הלכו את כולה, ורבי יהושע ורבי עקיבא לא זזו מארבע אמות, שרצו להחמיר על עצמן.

Rav Naḥman bar Yitzḥak said: The Mishnah is also precise in its implication that the tanna’im do not disagree with regard to a moving boat. The Talmud asks: From where is this implied? From that which is taught: There was an incident where all of these rabbis were coming from Pelandarsin, and their boat set sail on the sea on Shabbat, taking them out beyond their Shabbat limit. Rabban Gamliel and R' Elazar ben Azarya walked about the entire boat, while R' Yehoshua and R' Akiva did not move beyond 4 cubits, as they sought to be stringent with themselves.

אי אמרת בשלמא במהלכת לא פליגי ⁠— היינו דקתני ״רצו״, דילמא עמדה.

Rav Naḥman bar Yitzḥak explains: Granted, if you say that they do not disagree with regard to a moving boat, that is why it is taught that they sought to be stringent with themselves, i.e., they wished to practice stringency although they were under no obligation to do so, as they were concerned that perhaps the boat will stand, i.e., come to a stop.