In Yonah 2:4:
|ד וַתַּשְׁלִיכֵנִי מְצוּלָה בִּלְבַב יַמִּים, וְנָהָר יְסֹבְבֵנִי; כָּל-מִשְׁבָּרֶיךָ וְגַלֶּיךָ, עָלַי עָבָרוּ.||4 For Thou didst cast me into the depth, in the heart of the seas, and the flood was round about me; all Thy waves and Thy billows passed over me.|
השלחת אותי מצולה בלבב ימים -- שהוא מה שהשליכו אותו מן האניה אל הים
וְנָהָר יְסֹבְבֵנִי -- שם נהר בא אל המים המשוכים ומובדל מן הים שהמים שבו עומדים ונקוים אבל אז שהסערה שהתחוללה בים היה מן התהום שמי הים עצמו התרגשו ונשאו דכים למטה נקרא נהר כי אז לא עמדו המים רק נמשכו בעיגול סביב עצמם ויסבבוהו סביב סביב, ואז
כָּל-מִשְׁבָּרֶיךָ וְגַלֶּיךָ, עָלַי עָבָרוּ -- שלא צף על הגלים כמעשה דרבי עקיבא רק נפל בעומק הים מקום שהמים שוטפים למטה והגלים עברו עליו מלמעלה
This does not seem to me to be a description of a true undersea river. Rather, there is the tehom -- typically understood as the true, special, watery depths. These caused the storm and tempest. He was tossed out of the boat into the water, and then, a whirlpool dragged him down to the bottom -- נמשכו בעיגול סביב עצמם ויסבבוהו סביב סביב. It is called a river, according to Malbim, because it is moving water, as opposed to the yam which is standing water. But it is only called nahar because it moves, not because it looked and behaved exactly like a river.
Now, Rabbi Yair Hoffman, on Matav:
Yes, the "river" was separate from the yam, by its nature and source. But Malbim never said it was an actual river at the bottom, and therefore had to explain why the term "river" makes sense -- moving water, dragging him down in a whirlpool. And levav yamim does not seem to mean sea bed. He treats it as a separate step, that this is to the sea itself, the top of the sea, for he went from the ship into levav yamim. Only as a separate step was he drawn from the levav yamim to below, via the whirlpool. And the idea
There is a remarkable Malbim on the book of Yonah 2:4. The Malbim discusses how Yonah was caught in a river under the sea - where the waters of the river were separate from the ocean waters. Until this month - this was generally thought to be a physical impossibility.Below, however, we find an article written by Richard Gray - the science editor of the British newspaper the telegraph. The results are striking. The reader is urged to see the Malbim with his or her own eyes. The term utilized by the Malbim - “Levav Yamim” clearly means sea bed.
This does not correspond with the newly discovered phenomena at all. Well, there is the idea of separate flow. It seems to me to be a misreading of the Malbim, from someone excited to sea this "prophetic" Malbim fulfilled. As the article ends:
The Malbim, Rav Meir Leibish Ben Yechiel Michel Weiser passed away in 1879. The motif of his commentary was to prove the Divine origin of the Torah - as a counter measure to the rise of the Reform movement. The fact is that the commentary of the Malbim is filled with such gems.Whirlpools, meanwhile, are well-known phenomena.
Update: See also Yeranen Yaakov's take on it.