All the seas are like a mikveh [a gathering of waters], as it is said: "And the gathering of the waters He called Seas" - these are the words of Rabbi Meir. Rabbi Yehuda says: The Great Sea is like a mikveh, and it is not called "Seas" except because there are many kinds of seas within it.
Why the One Sea Is Called Seas in the Plural
Yalkut Shimoni on Torah 8:3
כָּל הַיַמִּים כְּמִקְוֶה שֶׁנֶּאֱמַר וּלְמִקְוֵה הַמַּיִם קָרָא יַמִּים, דִבְרֵי רַבִּי מֵאִיר. רַבִּי יְהוּדָה אוֹמֵר יָם הַגָּדוֹל כְּמִקְוֶה וְלֹא נֶאֱמַר יַמִּים אֶלָּא שֶׁיֵשׁ בּוֹ מִינֵי יַמִּים הַרְבֵּה.