आप एव यस्यायतनं हृदयं लोको मनो ज्योतिर्यो वै तं पुरुषं विद्यात्सर्वस्यात्मनः परायणं स वै वेदिता स्यात् ।याज्ञवल्क्य वेद वा अहं तं पुरुषं सर्वस्यात्मनः परायणं यमात्थ ।य एवायं अप्सु पुरुषः स एषः ।वदैव शाकल्य तस्य का देवतेति ।वरुण इति होवाच ॥३,९.१६ ॥
In plain words
"He whose dwelling is the waters, whose world is the heart, whose light is the mind — he who knows that person, the highest goal of every self, would truly be a knower, Yājñavalkya." "I do know that person, the highest goal of every self, of whom thou speakest. He is the person in the waters. Speak on, Śākalya. Which is his deity?" "Varuṇa," he said.
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
आप एव यस्य आयतनम् । साधारणाः सर्वा आप आयतनम् ; वापीकूपतडागाद्याश्रयासु अप्सु विशेषावस्थानम् ; तस्य का देवतेति, वरुण इति- वरुणात् संघातकर्त्तुः अध्यात्मम् आप एव वाप्याद्यपां निष्पत्तिकारणम् ॥
In plain words
Waters are his home, in the widest sense — every water there is. But look closer and it narrows to this: water gathered somewhere, held in a pond, a well, a tank. What god does he answer to? Varuna, the one who binds water into shape. And turned inward, it's simply water itself that gives rise to water — the pond is made of nothing but more water.
Close to the Sanskrit
'The waters alone are his abode' — all waters in general are his abode; but a particular location in waters residing in receptacles such as ponds, wells, tanks and the like is meant here. Of him, what is the deity? — 'Varuṇa', he said; from Varuṇa, the agent of binding together — with reference to the self, water itself is the cause of the arising of waters such as those in ponds and the like.