Another interpretation: people are astonished (mishtomemin) as to what they are made of, whether of water or of fire. From what is written, 'who lays the beams of His upper chambers in the waters' (Psalms 104:3), it follows that they are of water.
Whether the Heavens Are Made of Water or of Fire
Yalkut Shimoni on Torah 3:6
דָּבָר אַחֵר, שֶׁהַבְּרִיוֹת מִשְׁתּוֹמְמִין שֶׁל מָה הֵם, שֶׁל מַיִם אוֹ שֶׁל אֵשׁ, מִמַּה דִּכְתִיב "הַמְקָרֶה בַמַּיִם עֲלִיּוֹתָיו", הֲרֵי שֶׁל מַיִם הֵן.