Mishnah Mikvaos 5:4: All the seas are like a mikveh, as the verse says (Genesis 1:10) "And the gathering [literally: mikveh] of water, he called seas," according to Rabbi Meir. Rabbi Yehuda says: the great sea [i.e. the Mediterranean, alone] is like a mikveh; the verse only says "seas" [in the plural] because it contains many types of seas. Rabbi Yose says: all the seas purify [even] when they are flowing, and they are invalid for zavim, and for lepers, and to sanctify chatat waters from them.

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 מְטַהֲרִים בְּזוֹחֲלִין, וּפְסוּלִין לַזָּבִין וְלַמְצֹרָעִים, וּלְקַדֵּשׁ מֵהֶם מֵי חַטָּאת:

Mishnah Mikvaos 5:5: Flowing [water] sources are like a spring, and dripping [water] sources are like a mikveh. Rabbi Tzadok testified regarding flowing sources that [their waters] were greater than [those of] dripping sources [in a case where the two became mixed], that they are valid. And regarding a dripping source that was made into a flowing source [i.e. if it was somehow breached], one may put even a stick, or even a reed, or even a zav or a zavah beside it [the breach, in order to seal it off], and one may [then] descend and immerse [as into a mikveh of gathered waters], according to Rabbi Yehuda. Rabbi Yose says: anything which can be rendered impure, one cannot use it to stop the flowing.

הַזּוֹחֲלִין, כְּמַעְיָן. וְהַנּוֹטְפִים, כְּמִקְוֶה. הֵעִיד רַבִּי צָדוֹק עַל הַזּוֹחֲלִין שֶׁרַבּוּ עַל הַנּוֹטְפִים, שֶׁהֵם כְּשֵׁרִים. וְנוֹטְפִים שֶׁעֲשָׂאָן זוֹחֲלִין, סוֹמֵךְ אֲפִלּוּ מַקֵּל, אֲפִלּוּ קָנֶה, אֲפִלּוּ זָב וְזָבָה, יוֹרֵד וְטוֹבֵל, דִּבְרֵי רַבִּי יְהוּדָה. רַבִּי יוֹסֵי אוֹמֵר, כָּל דָּבָר שֶׁהוּא מְקַבֵּל טֻמְאָה, אֵין מַזְחִילִין בּוֹ: