Small boats of Mishan- shaped like a “top” in the sense that the width of 4 tefachim does not reach the floor of the boat- Rav Huna says they aren’t considered a reshus hayachid since the walls are “hanging ” bc they aren’t considered “walls” where the width is no loner 4 tefachim. Bc the walls are hanging and don’t reach the bottom of the domain, one cannot carry more than 4 Amos in the boat.

Rav Huna’s view is challenged by Rav Nachman who says the small boats are a reshus hayachid bc of the principle of גוד אחית- a Halacha from Sinai that walls automatically are viewed as extending downwards. If we use גוד אחית, the walls are viewed like they extend to the bottom of the reshus even though the floor is not 4 tefachim.

There’s a dispute between r Yosi and the rabanan if גוד אחית is destroyed by a physical reality that clearly indicates there is no wall, like goats breaking right through the imaginary wall. In our case with the small “top” boats, that point isnt relevant bc there’s nothing breaking through the imaginary extension of the walls in the water. Even though fish swim through the imaginary walls, the fish don’t break the imaginary wall. (They aren’t visible)

An illustration of these points: In an old ruined building, where the bottom of the walls are gone so the walls don’t reach the floor, R Yosi says we use גוד אחית and the interior is a reshus hayachid but the rabanan say the goats walking right into the ruin takes away the usage of גוד אחית. However, in water there is no problem with fish breaking imaginary walls so someone can draw water using walls that suspend over water even like the rabanan.

One cannot carry between two seperate reshus hayachids owned by different people on Shabbos without an Eiruv Chazterios that enjoins the two domains. An eiruv only works for two adjacent reshus hayachids, but not between two reshus hayachids separated by a karmelis.

Because of these issues, two boats floating in the sea owned by dif people need an eiruv chatzeiros to carry between them. The eiruv is only effective if the boats are tied bc otherwise the sea will separate beteeen them. They can be tied with any material as long as the boats will not separate from each other.