The Gemara on today’s daf discusses the laws of transferring objects on and off a boat on Shabbat.
It was stated that the Sages disagreed with regard to the manner in which one may draw seawater onto a boat on Shabbat. Rav Huna said: One extends a projection of any size from the side of the boat as a distinctive sign, and fills a receptacle with water from the sea. Rav Ḥisda and Rabba bar Rav Huna say: One creates an area, a frame of four by four handbreadths, and fills the water from inside it.
The Gemara explains that the source of this disagreement is whether we measure from the sea floor or from the water. According to Rav Huna, we measure from the sea floor. Since the sea itself is deeper than ten handbreadths, the boat is considered to be floating in the air, and the air is an “exempt domain,” so transfer is permitted. According to Rav Ḥisda and Rabba bar Rav Huna we measure from the water and the water in the sea has a legal status of solid land. Therefore, if one does not create an area of four by four, he will be carrying from the sea (a karmelit) to the private domain.
The early commentaries on the Talmud discussed this halakha and expressed several opinions on the matter. According to the Rambam, there is no need for partitions. It is sufficient to extend a beam that is four by four handbreadths with a hole carved in it. Rashi requires that the beam must have small partitions. This opinion is shared by the ge’onim based on the opinions of most of the amora’im in the Jerusalem Talmud. Some explain that this requires a beam that is four by four cubits with a hole in it that is four by four handbreadths (Rabbeinu Tam cited in the Rashba). Some commentaries even assert that the legal status of the area in the boat from which water is drawn to the boat is like that of a partition hanging from a balcony. The partitions surrounding that area must be ten handbreadths high (Me’iri).