When there was a complete inundation after the period of the Cākṣuṣa Manu and the whole world was deep within water, the Lord accepted the form of a fish and protected Vaivasvata Manu, keeping him up on a boat.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
रूपं स जगृहे मात्स्यं चाक्षुषोदधिसम्प्लवे । नाव्यारोप्य महीमय्यामपाद्वैवस्वतं मनुम्
rūpaṃ sa jagṛhe mātsyaṃ cākṣuṣodadhisamplave | nāvyāropya mahīmayyāmapādvaivasvataṃ manum
SB 1.3.15