आपो वै नरसूनवः ।ता यदस्यायनं पूर्वं तस्मान्नारायणः स्मृतः' इति मनुवचनाद्वा नारायणः ।
In plain words
"The waters are the children of Nara. Because they were his first dwelling, he is called Nārāyaṇa" — so Manu says; and on the strength of those words too, the name is Nārāyaṇa.
Close to the Sanskrit
"The waters, indeed, are the offspring of Nara. Since they were his first abode, therefore he is remembered as Nārāyaṇa" — thus, from Manu's statement, alternatively, [he is] Nārāyaṇa.