Mikvaos, Chapter 6, Mishnah 5. This chapter deals with the joining of bodies of water and with the status of water that sits inside a vessel. Our mishnah applies both themes to a striking case: vessels floating in the sea. Water inside a k'li cannot serve for tevilah, since it has the status of water gathered in a vessel. The question the Mishnah asks is how much of an opening is needed before we stop viewing that water as vessel water and begin viewing it as part of the sea itself.
"Hashida v'hateiva sheb'yam." A shida and a teiva are both varieties of oversized chest. Picture them resting in the ocean with their upper edges rising above the surface, so that whatever water stands within is hemmed in by the wooden walls and does not lie open to the sea around them. Concerning such water our mishnah rules: "ein matbilin bahen," keilim may not be dipped in it. There is one exception: "ella im kein hoyu nekuvin kishfoferes hanod," provided a perforation connects the chest to the sea, and that perforation is as wide as a shfoferes hanod, the spout of a wineskin.
Rebbi Yehuda disputes that measurement in this instance, holding that the requirement varies with the dimensions of the box itself. "Bikli gadol": where the vessel is large, meaning it measures more than eight tefachim, the gap must reach four tefachim; anything narrower leaves the enclosed water unattached to the sea. "U'v'katon": where the vessel is small, under eight tefachim, the standard is "ruboy," meaning that most of the wall must be broken away. Short of that, the water within is not deemed to have merged with the water outside.
The mefarshim raise an obvious difficulty. Earlier in the tractate Rebbi Yehuda himself accepted the measure of a shfoferes hanod as sufficient to join one body of water to another. Why does he demand so much more here?
The resolution lies in recognizing that two unrelated problems are under discussion. In the earlier case the water sat in a spot entirely suitable for a mikveh; its only deficiency was that it lacked forty sa'im on its own. For that, contact through a shfoferes hanod suffices, producing an eiruv mikvaos in which the two pools count as a single body of water. A chest floating in the sea, by contrast, is no proper site for a mikveh at all, because a shida or a teiva is a k'li. Joining the waters therefore accomplishes nothing; the very identity of the vessel has to be cancelled. Cancelling it demands an opening of four tefachim in a sizable box, or the loss of most of one wall in a small one.
The Mishnah now shifts to a completely different sort of container. "Im hoyso sak o kupo": suppose the object sitting in the sea is a sack woven of rough fabric, or a wicker hamper, a k'li shot through with gaps. In that case, "matbilin bahem k'mo sheheim," immersion inside them is valid with no modification whatsoever, "mipnei shehamayim m'oravin": the water passes freely back and forth. Its countless perforations knit the water within to the sea surrounding it, so that dipping an object in the basket amounts to dipping it in the mikveh proper. That is exactly how we handle things nowadays: an item requiring tevilah is set into a basket and the basket is lowered into the mikveh, because the openings in its weave keep the water inside and the water outside as one.
"Hoyu n'sunim tachas hatzina." What if such a sack or basket was placed underneath the pipe that channels rainwater into the mikveh, so that the incoming water passed through it on its way in? "Einam poslin es hamikveh," they do not invalidate the mikveh. Water becomes she'uvim only when it is niskabel b'kli, received in a vessel, and these keilim are so porous that the water was never truly held in them. It merely ran through. Therefore the water that arrives in the mikveh retains its status as rainwater.
"Ella matbilin osan u'ma'alin osan k'darkon." Beyond permitting immersion in them, the Mishnah allows them to be removed from the water in the usual fashion. The contrast with a solid, watertight k'li makes the point clear. Where a mikveh holds precisely forty sa'im and nothing more, a vessel must be drawn up inverted, so that its contents spill back and are never treated as drawn water. Were it raised upright instead, the water trapped within becomes she'uvim, may no longer be poured back, and the mikveh is left below its required measure. Sacks and baskets, perforated throughout, never raise any she'uvim question about the water they hold, and so they may be brought up out of the water in the normal manner, with no precautions whatsoever.
The mishnah thus draws a clean line. A closed box in the sea remains a k'li, and its water remains vessel water until the walls are breached enough to strip away that status. A sack or a basket, whose walls barely separate inside from outside, never confines water in the first place, and so it may be used freely for tevilah, may stand under the incoming stream, and may be lifted out just as it is.