Parah, Chapter 8, Mishnah 8. A new subject opens here, and the rest of the mishnayos in our perek are devoted to defining mayim chayim, living water, that is, the water of a spring, which alone may be used in preparing the chatas water together with the ashes of the parah adumah. Our mishnah's question is how to classify the seas: does halachah regard them as a mikveh, or as a spring?
Why should this matter so much? Because there are purifications that may be carried out with living water and with nothing else: sanctifying the chatas water, a zav's immersion, and the ritual of the two birds by which a metzora is purified. Beyond those cases, a mikveh and a spring part ways on a fundamental point. A mikveh is effective only where its water has collected and is standing in place, while spring water works even while it runs along, zochalin.
Rebbi Meir: all the seas are a mikveh
The mishnah begins with Rebbi Meir's ruling: "Kol hayamim kemikveh, shene'emar, ulemikveh hamayim kara yamim" - every sea carries the halachic status of a mikveh, for the verse states, "and the gathering together of the waters He called seas." Such is the teaching of Rebbi Meir, divrei Reb Meir.
On this view, every sea is governed by the halachos of a mikveh rather than by those of a spring. It follows that the chatas water may not be sanctified from any sea, that a zav may not immerse in one, and that the birds of a metzora may not be brought with sea water, since each of those procedures calls for living water from a spring. As for anyone else who has contracted tumah and comes to immerse, a sea serves him as a mikveh does: it is effective at a spot where the water lies at rest, and ineffective where the water is running.
Rebbi Yehudah: only the Great Sea
Rebbi Yehudah narrows the ruling considerably: "Hayam hagadol kemikveh", it is the Great Sea, the ocean, that holds the status of a mikveh. "Lo ne'emar yamim", the plural form in the verse was not chosen in order to include every sea on earth; Scripture is speaking of the Great Sea by itself, the body of water toward which all the rivers make their way, "ela sheyeish bo" (in that one great ocean) "minei yamim harbei", since it contains within itself many different varieties of sea.
Rebbi Yosi: they purify even while flowing, yet remain unfit for mayim chayim
A third position is that of Rebbi Yosi: "Kol hayamim metaherin bezochalin", all the seas accomplish purification even where their water is on the move and not only where it stands, and in this respect they behave like a spring. Even so, "ufsulin lezavim, velimetzora'im, ulekadesh mehen mei chatas": a zav may not immerse in them, a metzora's birds may not be brought with their water, and the chatas water may not be sanctified from them, because those three require genuine living water welling up from a spring.
So our mishnah leaves us with three views on the standing of the seas, and with the principle that will guide the rest of the chapter: the chatas water may be sanctified only with living water, and we must now learn exactly which waters earn that name.