Another interpretation: 'Lift up (se'u) the waters' means 'laden with water.' A parable: like milk placed in a bowl. So long as no drop of rennet has fallen into the bowl, it quivers; but once a drop of rennet falls into it, at once it congeals and stands firm. So too 'the pillars of heaven tremble' (Job 26:11), as in what Rav said: the heavens were moist on the first day, and on the second they congealed.
The Heavens That Set Firm Like Milk Turning to Curd
Yalkut Shimoni on Torah 3:8
דָּבָר אַחֵר, שָׂא מַיִם, טָעוּן מַיִם מָשָׁל לְחָלָב הַנָּתוּן בִּקְעָרָה, עַד שֶׁלֹּא תֵּרֵד בַּקְּעָרָה טִפָּה שֶׁל מָסוּ הוּא מְרַפְרֵף כֵּיוָן שֶׁיָּרַד לְתוֹכָהּ טִפָּה שֶׁל מָסוּ מִיָּד קוֹפֵא וְעוֹמֵד, כָּךְ עַמּוּדֵי שָׁמַיִם יְרוֹפְפוּ כְּהַהִיא דְּאָמַר רַב, לַחִין הָיוּ שָׁמַיִם בְּיוֹם רִאשׁוֹן וּבַשֵּׁנִי קָרְשׁוּ.