Parah, Chapter 9, Mishnah 6. This chapter has been dealing with the many ways in which the chattas water (the water mixed with the ashes of the parah adumah) can be spoiled or disqualified. Our mishnah turns to a different sort of restriction: how these sacred materials may, and may not, be moved from place to place when a river lies in the way.
The words of the mishnah run as follows. "Mei chattas ve'eifer chattas", the water of the chattas together with the ashes of the chattas: "lo ya'avirem banahar uvesfinah", one is not to bring them over a river by boat. "Velo yashitem", nor is he to set them floating "al pnei hamayim", upon the surface of the water. "Velo ya'amod betzad zeh", nor may he take up a position on one bank "vizrokem letzad zeh", tossing them over to the bank opposite.
Then comes the leniency: "Aval over hu vamayim", he himself, however, may make his way across on foot through the river, "ad tzavaro", even where the water rises as high as his throat.
The closing line adds: "Over hu hatahor lechattas", a man who has been purified to the standard demanded for the chattas may make the crossing, "uveyado kli reikam", with an empty utensil in his hand that is itself "hatahor lechattas", fit by that same standard; "uvamayim she'einam mekudashin", and so too he may cross carrying water into which the ashes have not yet been placed. The mishnah's restriction, in other words, reaches only the ashes themselves and water already sanctified with them. A pure empty vessel, or ordinary water not yet turned into chattas water, presents no difficulty whatsoever.
Why not a boat?
The craft under discussion is the ferry that brings travelers and their loads from one bank of a river to the other. The Gemara in Chagigah supplies the background to the ruling: a man once brought the sanctified water along with the ashes over the Jordan on such a ferry, and afterward an olive's volume of flesh from a corpse was found lying inside that very boat. Once that had happened, the chachamim directed that these sacred materials no longer be taken over that river aboard a ship.
Floating and throwing
The mishnah extends the ruling. One may not set the chattas water or ashes afloat in a container and let them ride across the surface of the river, since that is essentially the same act as sending them across in a boat. So too, standing on one side of the river and hurling them across to the far bank is treated as the equivalent of a boat crossing, and is likewise forbidden.
Wading through the water
By contrast, a person who carries the chattas water or ashes himself and walks through the river on his own feet is permitted to do so. This is true even when the water comes right up to his neck, so that from a distance it appears as though he and his burden are floating along on the water. Since he is crossing under his own power and not being carried, this is nothing like traveling by boat, and it is entirely permissible.