RADAK
1.
For He has founded it upon (the) seas, And established it upon (the) floods:
The earth He has established upon seas and floods for the sake of the creatures upon it; but this mountain He has not founded for all creatures, not even for all the sons of men, but for the clean of hand and pure of heart; and it is the seed of Jacob who seek the Lord, and were bidden to be clean of hand and pure of heart.
2.
And the interpretation of upon (the) seas and upon (the) floods is "next to" seas and rivers.
3.
And (it is said) that there is no habitable spot in the world (more than) eighteen days' (journey) distant from the sea, and this is for the provision of moisture for the creatures;
and likewise the rivers are for the use (provision) of the creatures; and this is according to what is said (Yeshayahu - Isaiah 45:18), "He formed it to be inhabited" because He established it so that created beings might live in it.
And the reason (for so doing) was because it had not been created so from the beginning, but rather (at first) to be entirely under water; for the waters encompassed it on every side as the air encompassed the waters on every side, and the fire encompasses the air on every side. And behold the earth was under the waters, in accordance with the nature He imposes on them; and He did not make it so at first, in order to demonstrate that all is in His hand and that He varies (their) natures at His pleasure. And God commanded that its first division should be uncovered, so that the creatures might live in it; and (so) He says (Bereshit - Genesis 1:9) "Let the waters under the heaven be gathered unto one place and let the dry land appear."