The Midrash Tanchuma in Vayikra discusses the wonders shown to Jonah by the fish which swallowed him after having saved it from the Leviathan (see the Midrash [ibid.] for further elaboration).
(Source from Sefaria.org)
אמר לו לדג, הריני הצלתיך מפיו של לויתן, הראני כל מה שבים ובתהומות. והראהו נהר גדול של מימי אוקינוס, שנאמר: תהום יסובבני (יונה ב, ו)
[Jonah] said to the fish, "behold, I saved you from the mouth of the Leviathan, show me all that is in the sea and its depths", and [the fish] showed him the great river of the waters of the ocean, as it is written "the depths surrounded me" (Jonah 2:6)
My Question:
What is "the great river of the waters of the ocean?" is it discussed anywhere else in the Midrashim describing its location/function?