אלא במים קל הוא שהקילו חכמים במים ואמאי הא איכא בקיעת דגים אלא שמע מינה בקיעת דגים לא שמה בקיעה:

in water. It is a leniency the Sages instituted in water but not in other circumstances. And why were they lenient with regard to a hanging partition in water? Isn’t there the passage of fish? Rather, learn from this that the passage of fish is not considered passage.

ספינות קשורות כו׳: פשיטא אמר רבא לא נצרכה אלא להתיר ביצית שביניהן

We learned in the mishna: If boats are tied together, one may carry an object from one to the other on Shabbat. The Gemara asks: That is obvious, since these boats are like a single domain. Rava said: This mishna was necessary only to permit carrying from one boat to another via a small boat that is between them.

אמר ליה רב ספרא משה שפיר קאמרת מטלטלין מזו לזו תנן אלא אמר רב ספרא לא נצרכה אלא לערב ולטלטל מזו לזו וכדתניא ספינות קשורות זו בזו מערבין ומטלטלין מזו לזו נפסקו נאסרו חזרו ונקשרו בין שוגגין ובין מזידין בין אנוסין בין מוטעין חזרו להיתרן הראשון

Rav Safra said to him: You, who are as great in this generation as Moses, did you speak well? We learned in the mishna that one may carry only from one to the other, not via a small boat. Rather, Rav Safra said: The mishna was only necessary to obligate one to place an eiruv, a joining of courtyards, between the two boats. Since the boats belong to different people, they must be joined to form a single domain in order to permit carrying from one to the other, as it was taught in a baraita: With regard to boats tied to one another, one places an eiruv and carries from one to the other. If the ties between them were severed, the people on the boats are prohibited to carry from one to the other. If they were then retied, whether unwittingly, i.e., the one who retied them forgot that it was Shabbat, whether intentionally, whether due to circumstances beyond one’s control, whether mistakenly, the boats are restored to their original permitted status.

וכן מחצלות הפרוסות (לרשות הרבים) מערבין ומטלטלין מזו לזו נגללו נאסרו חזרו ונפרשו בין שוגגין בין מזידין בין אנוסין ובין מוטעין חזרו להיתרן הראשון שכל מחיצה שנעשה בשבת בין בשוגג בין במזיד שמה מחיצה

And similarly, in the case of mats that are unfurled to create a partition between two people and the public domain, one places an eiruv and carries from one to the other. If the mats were furled, the people on the boats are prohibited to carry from one to the other. If the mats were then unfurled again, whether unwittingly, whether intentionally, whether due to circumstances beyond one’s control, whether mistakenly, they are restored to their original permitted status. That is because any partition that is established on Shabbat, whether unwittingly, whether intentionally, is considered a partition.

איני והאמר רב נחמן לא שנו אלא לזרוק אבל לטלטל אסור כי איתמר דרב נחמן אמזיד איתמר

The Gemara asks: Is that so? Didn’t Rav Naḥman say: They only taught the principle that a partition established on Shabbat is considered a partition with regard to throwing. In that case, a partition creates a domain unto itself, and one who throws an object into it from another domain is liable. However, with regard to carrying within that domain, it is certainly prohibited. The Gemara answers: When that statement of Rav Naḥman was stated, it was stated with regard to an act performed intentionally. One who intentionally establishes a partition is penalized and is not permitted to benefit from it. In principle, though, that partition is considered a full-fledged partition.

אמר שמואל ואפילו קשורות בחוט הסרבל היכי דמי אי דיכול להעמידן פשיטא אי דאין יכול להעמידן אמאי

Shmuel said: The halakha that one may carry from one ship to another if they are tied together applies even if they were tied with a string used to close the neckline of a cloak. The Gemara asks: What are the circumstances? If the string is capable of holding the ships together, it is obvious that carrying between the ships is permitted as they are tied together. However, if the string is incapable of holding them, why is it permitted?

לעולם דיכול להעמידן ושמואל לאפוקי מדנפשיה קאתי דתנן קשרה בדבר המעמידה מביא לה טומאה בדבר שאין מעמידה אין מביא לה טומאה ואמר שמואל והוא שקשורה בשלשלת של ברזל

The Gemara explains: Actually, it refers to a string that can hold them, and Shmuel said this to exclude this case from his own statement. As we learned in a mishna: If one tied a ship with an item capable of holding it and the end of that item was in a tent with a corpse, it transmits impurity to the ship. And if one tied it with something that is incapable of holding it, it does not transmit impurity to the ship. And Shmuel said: When the mishna refers to an item capable of holding it, it is referring to a case where it is tied with an iron chain.

לענין טומאה הוא דכתיב בחלל חרב חרב הרי הוא כחלל אין אבל לענין שבת כיון דיכול להעמידה (היכר בעלמא הוא) אפילו בחוט הסרבל:

It was necessary for Shmuel to establish that although with regard to ritual impurity the halakha applies only to an iron chain, with regard to Shabbat the halakha applies to any item capable of holding the ships together. The reason that the halakha is different with regard to impurity is as it is written: “And whoever touches in the open field one slain by sword, or one who dies by himself, or a bone of a man, or a grave, shall be unclean seven days” (Numbers 19:16). The Sages derived from the phrase: One slain by sword that a sword is like one slain, i.e., a corpse. A metal instrument that comes into contact with a corpse assumes the same level of ritual impurity as the corpse itself, the ultimate primary source of ritual impurity. Therefore, it is only an iron chain in a tent with a corpse in it that can render a boat tied to the other end a primary source of ritual impurity. A string made of other materials cannot. However, with regard to Shabbat, since it is capable of holding it and it is a mere distinctive sign that is necessary, even the string of a cloak is sufficient.