Mikvaos, Chapter 5, Mishnah 4. This chapter examines the various natural bodies of water that exist in the world and asks, about each one, which set of laws it follows: is it a mikveh, a gathering of rainwater, or is it a ma'ayan, a spring? Our mishnah turns to the largest bodies of water of all, the seas and oceans.
Everything here rests on a verse in parshas Bereishis. When the waters of the world were gathered together, the Torah tells us that a name was assigned to them: "ul'mikveh hamayim kara yamim", the assembled waters were designated as seas. Chazal saw in this far more than a label. The posuk sets the term mikveh alongside the term yamim, and from that pairing they derived that a sea shares in the halachic character of a mikveh. Precisely what it shares is what the mishnayos before us come to define.
The View of Rabi Meir
The mishnah opens: "kol hayamim k'mikveh" - all the seas have the status of a mikveh. Whatever body of water we are speaking about, the Mediterranean, the Atlantic, the Pacific, its halacha is that of a gathering of rainwater. Two practical consequences follow. First, it is eino metaheir bezochlin: it does not purify while the water is in motion, so water running off from the sea cannot be used for immersion. Second, it does not have the status of mayim chayim, living water. Naturally there is no question at all about the requirement of forty se'ah, since a sea contains that measure many times over.
And the source? "Shene'emar ul'mikveh hamayim kara yamim" - the posuk itself attaches the word mikveh to yamim, so a sea is to be treated as a mikveh. This, the mishnah tells us, is divrei Rabi Meir, the opinion of Rabi Meir.
The View of Rabi Yehuda
"Rabi Yehuda omer: hayam hagadol k'mikveh" - Rabi Yehuda narrows the ruling considerably. It is only the Great Sea whose din is that of a mikveh. What singles it out? It is the sea into which every other body of water eventually drains. The posuk's identification of a mikveh with yamim was said about that sea exclusively. Every other sea, and every other natural collection of water in the world, is governed instead by the laws of a ma'ayan.
Rabi Yehuda finds his distinction in the precise language of the verse. The Torah did not write the word in the singular; it wrote yamim, in the plural, and the plural indicates a sea that contains within it "minei yamim harbei", many varieties of seas. Only the sea that gathers in the waters of all the rest answers to such a description.
The View of Rabi Yosi
"Rabi Yosi omer: kol hayamim metaharin bezochlin" - Rabi Yosi holds that all the seas purify even while their water is flowing, because in essence they carry the halacha of a ma'ayan. If so, what did the Torah come to teach by calling the gatherings of water yamim? For this: "ufsulim lezavin ul'metzoraim ul'kadesh meihem mei chatas" - the seas are invalid for a zav, for a metzora, and for the sanctifying of the waters of the parah adumah. In each of those three areas the Torah demands fresh spring water, mayim chayim, and the posuk comes to disqualify the sea by teaching that its water does not have the status of mayim chayim.
But regarding every other halacha of a ma'ayan, that it is valid even with less than forty se'ah, and that it purifies while the water is running, Rabi Yosi holds that a sea is treated as a full ma'ayan.