God Nārāyaṇa, assuming the Self of Brahman (the Supreme Divinity), was lying on water — upon his anantaśayyā (anantasayya), and that is why he is referred to as Āponārā (Aponara) —
āpo nārā iti proktā āpo vai narasūnabaḥ
ayanaṃ tasya tā yasmāt tena nārāyaṇaḥ smṛtaḥ.