Mishnah Parah 8:7: Anything which [is sufficiently impure such that it] invalidates trumah [a portion of produce which must be given to a priest and is not allowed to become ritually impure] upon contact makes liquids impure [to the extent] that they become a 'primary' [source of impurity] to make one thing impure, and [then] to invalidate another. [The intensity of ritual impurity depends on how many steps removed it was from the original source. Our Mishnah teaches that when a liquid becomes impure by coming into contact with something impure, it is considered 'primary' to the source of impurity, meaning that any liquid or food it comes into contact with then becomes impure as a 'secondary' source of impurity, which in turn is sufficient to impurify and therefore invalidate trumah with which it comes into contact.] The exception to this is one who immersed [for purification] that day and is yet to have the sun set [for such an individual invalidates trumah upon contact but does not make liquids he touches into 'primary' sources of impurity]. Behold it [is as though the food] says [to the liquid]: That which renders you impure [i.e. something of a 'secondary' level of impurity] does not impurify me, yet you [the liquid, after becoming impure through contact with that impurity of a 'secondary' level] have rendered me impure!"

כָּל הַפּוֹסֵל אֶת הַתְּרוּמָה, מְטַמֵּא אֶת הַמַּשְׁקִין לִהְיוֹת תְּחִלָּה לְטַמֵּא אֶחָד וְלִפְסֹל אֶחָד, חוּץ מִטְּבוּל יוֹם. הֲרֵי זֶה אוֹמֵר, מְטַמְּאֶיךָ לֹא טִמְּאוּנִי וְאַתָּה טִמֵּאתָנִי:

Mishnah Parah 8:8: All the seas are like a mikvah [a collection of water in which one can immerse in order to remove certain ritual impurities], as it is Said (Genesis 1:10), "And the gatherings [literally: mikvahs] of water, he called seas," these are the words of Rabbi Meir. Rabbi Yehuda says: [Only] the great sea [i.e. the Mediterranean] is like a mikvah. It only says "seas" [in the plural] regarding something contains many types of seas [i.e. the ocean, into which flow many bodies of water]. Rabbi Yose says: All seas purify [as a mikvah] with [even only] a gradual flow [of water], yet they are invalid for a zav [an individual who is ritually impure due to having had a type of seminal emission, and who must immerse in naturally flowing waters as part of the purification process], a metzorah [one who is ritually impure due to having a type of skin disease, and who must have a bird sacrificed above flowing waters as part of his purification process], and to sanctify the waters of a chatat [the red heifer ritual, which also must come from flowing waters; Rabbi Yossi says that seas are not a valid source for these three rituals, which require use of naturally flowing waters].

כָּל הַיַּמִּים כְּמִקְוֶה, שֶׁנֶּאֱמַר (בראשית א), וּלְמִקְוֵה הַמַּיִם קָרָא יַמִּים, דִּבְרֵי רַבִּי מֵאִיר. רַבִּי יְהוּדָה אוֹמֵר, הַיָּם הַגָּדוֹל כְּמִקְוֶה, לֹא נֶאֱמַר יַמִּים, אֶלָּא שֶׁיֶּשׁ בּוֹ מִינֵי יַמִּים הַרְבֵּה. רַבִּי יוֹסֵי אוֹמֵר, כָּל הַיַּמִּים מְטַהֲרִין בְּזוֹחֲלִין, וּפְסוּלִין לַזָּבִים וְלַמְצֹרָעִים וּלְקַדֵּשׁ מֵהֶן מֵי חַטָּאת: