What was very far?
Rashi: The water ceased far from where they stood.
Radak: It was very far from the closest city (Adam ha'Ir) even after it piled up, for it piled straight up (did not go to the sides).
Malbim: It was very far from the people of the city Tzarasan (so they were not harmed).
What is "me'Adam ha'Ir"?
Rashi: This is the city's name.
Malbim: It is the people of the city [Tzarasan].
Why is it written ba'Adam? We read it like me'Adam!
Radak: Ba'Adam hints that had the water not piled up, rather, spread to the sides, it would have entered the city Adam and flooded it, even though it was far away.
What is the meaning of "Tamu Nichrasu"?
Rashi: The water flowed like rivers normally do, to the sea, until [the water] finished.
Radak: The water below from where it divided flowed normally, and before they reached there, it was finished (all went to the sea), and they walked on the dry [river floor] on a wide path.
Malbim: The water [below], since it was cut off [from the water that piled up, flowed normally] and was finished.
How could water pile up? Miracles do not cause things to oppose their natures, e.g. a round circle or a diagonal equal to the side!
Ralbag (4:20): This is not considered against its nature, for there are thick transparent things that inhibit water from falling, or a strong wind. 1
Malbim (3:17): Hashem does miracles that change natures, as long as it is not an inherent contradiction, e.g. a round circle. And even if you will say that He does not change natures, it is not water's intrinsic nature to fall, just gravity pulls it.
Malbim: If there was a strong wind, it would have scattered the rising water! If something physical stopped the water from falling, the Kohanim would have sensed it!