The abstract noun "fullness" can be stated with the verb "fills." AT: "everything that fills it" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])
Here "founded it upon the seas" and "established it on the rivers" mean basically the same thing. The Hebrews of that time believed their land was supported by the oceans and deep underground rivers. AT: For he formed its foundation on the seas and built it on the deep waters" (See: [[rc://en/ta/man/translate/figs-parallelism]] and [[rc://en/ta/man/translate/figs-explicit]])