1) THE SMALL BOATS ON THE MEISHAN RIVER
QUESTION: The Gemara states that one may not carry an object more than four Amos in small boats (canoes, according to RASHI) on Shabbos, because the boat becomes four Tefachim wide only above a height of three Tefachim from the floor of the boat, and thus the boat is not considered a Reshus ha'Yachid.
Even though the boat is not a Reshus ha'Yachid, it should also not be considered a Karmelis, because a Karmelis also must be four Tefachim wide. Rather, the boat should be considered a Makom Petur, and it should not be forbidden to carry within it! (TOSFOS DH Hani)
(a) The RITVA answers that the Rabanan consider an area a Karmelis even if it does not have walls that are four Tefachim apart that reach the ground, even though the walls must reach the ground in order to be defined as a Reshus ha'Yachid. The SEFAS EMES adds that although a Karmelis must have an area of four by four Tefachim, it does not need Mechitzos that enclose the area in order for it to be a Karmelis.
(b) The RITVA quotes his Rebbi, the RE'AH, who says that, indeed, even in a Makom Petur one is forbidden to carry an object four Amos! Even though we find that one is permitted to carry from a Makom Petur to a Reshus ha'Yachid or to a Reshus ha'Rabim, and from those domains to a Makom Petur, one is permitted only to transfer an object between domains. Within the Makom Petur itself, though, one is forbidden to carry four Amos.
(c) Since the boat constantly moves, it is not considered to be a Reshus of its own (a Makom Petur). Rather, it is subordinate to the sea on which it floats, which is a Karmelis. Hence, the boat is a Karmelis and one may not carry four Amos in a Karmelis. However, if the boat would have the proper dimensions to be considered a Reshus ha'Yachid, it would not be subordinate to the sea, because a Reshus ha'Yachid is always significant unto itself. (M. KORNFELD, based on the RASHBA to 5a)
(d) TOSFOS argues with Rashi and gives a completely different explanation for the Gemara. The Gemara is not discussing a canoe, but rather a floatation device that looks like an open raft or dinghy. The middle of the raft is open and has no floor. It is partially submerged, and people sit partially submerged on the bench-like perimeter of the raft. The raft itself is more than four by four Tefachim wide. (In accordance with his definition of the small boat, Tosfos understands many points in the Gemara completely differently).
2) THE BREACH OF FISH
QUESTION: The Gemara explains that the presence of fish that swim under a wall that hangs over a body of water is not considered to breach that wall. Even the area under the wall is still considered to be an extension of the wall ("Gud Achis"). The Gemara cites proof for this from a statement of Rav, who says that a "hanging Mechitzah" (a wall that does not reach the ground) is not considered to extend downward except when it is over water, and this is a "leniency with regard to water which the Chachamim permitted." It must be that the fact that fish swim under the wall does not invalidate the presence of the Halachic extension of the wall.
If the presence of fish is not considered a breach in the Halachic wall, then why does Rav call this a "leniency with regard to water"? It is not a leniency, but rather it is m'Ikar ha'Din!
ANSWER: The RITVA answers that, in truth, the presence of fish should be considered a breach in the Halachic wall, for two reasons. First, the presence of fish should be no different than the presence of any other animal (such as goats, and "Gediyim Bok'in Bah" does create a breach in a Mechitzah (Eruvin 14a). Second, fish swim under the hanging wall more frequently and with greater ease than animals that crawl under a wall that hangs over land. Nevertheless, the Chachamim were lenient and applied the rule of Gud Achis because fish are not visible, as Rashi mentions here.