Comentario sobre Pará 9:6

מי חטאת – we are speaking of mixed purification waters. But waters that are not mixed are permitted, as it is taught in the concluding clause [of the Mishnah].

Hatat waters and hatat ashes one may not carry them across a river on a ship, nor may one float them upon the water, nor may one stand on the bank on one side and throw them across to the other side. According to traditional explanations of the prohibition in this section (which are based on the Bavli) it occurred one time that a person was transporting hatat waters and hatat ashes on a boat to cross the Jordan River. A piece of corpse was found on the bottom of the boat, and that caused the waters and ashes to be defiled and thereby disqualified. At that time the sages decreed that one should not carry hatat waters or ashes over water in a boat, nor transport them over a river in other similar ways. I must admit that I find these types of explanations somewhat intellectually dissatisfying. Just because an accident happens once doesn't mean it is any more likely to occur again. In my opinion, when the Talmud explains laws in this ways, it is in essence admitting that the law is irrational. It is as if they are saying, we have no real explanation for this, so it must be that the law was simply created when such a case actually occurred.

ואפר חטאת – even the ash itself without water.

One may, however, cross over with the water up to his neck. However, it is permitted to carry the water across by hand, even if he is up to his neck in water. This was not considered similar enough to sailing on a ship for the rabbis to have prohibited it due to the incident described above.

לא יעבירם בנהר ובספינה – that a decree was made because of an incident of one person who was taking the waters of purification and the ashes of purification in a boat on the Jordan [River] and it was found that an olive’s bulk of a corpse was inserted in the bottom of the boat.

He that is clean for the hatat may [sail] across [a river] carrying in his hands an empty vessel that is clean for the hatat or water that has not yet been prepared. A person who is ritually clean enough to prepare the hatat waters can travel on a ship while holding a vessel that will be used for the mixture and with water into which the ashes have not yet been mixed. The incident that triggered the prohibition occurred with waters that had already been mixed and with hatat ashes therefore that is the only case that they prohibited.

ולא ישיטם על פני המים (he should not float them on the water) – for it is similar to a boat, and similarly, he should not stand on this side of the river and cast them to the other side, and also that this is similar a bit to a boat.

אבל עובר הוא – on his feet.

במים – with the waters of purification that are in his hand.

עובר הוא הטהור לחטאת – whether in a boat, whether he floats over the water a vessel which is pure/clean to place in it the waters of purification when they are empty, or that he has water in it water that had not yet been mixed with the ashes of the [Red] Heifer. For they did not decree other than on the mixed waters and on the ashes alone, not on the man or the pure vessels (and the waters that are not mixed).