SB 1.3.15

rūpaṁ sa jagṛhe mātsyaṁ
 cākṣuṣodadhi-samplave
nāvy āropya mahī-mayyām
 apād vaivasvataṁ manum
Synonyms: 
rūpam — form; saḥ — He; jagṛhe — accepted; mātsyam — of a fish; cākṣuṣa — Cākṣuṣa; udadhi — water; samplave — inundation; nāvi — on the boat; āropya — keeping on; mahī — the earth; mayyām — drowned in; apāt — protected; vaivasvatam — Vaivasvata; manum — Manu, the father of man.
Translation: 
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.
Purport: 

According to Śrīpāda Śrīdhara Svāmī, the original commentator on the Bhāgavatam, there is not always a devastation after the change of every Manu. And yet this inundation after the period of Cākṣuṣa Manu took place in order to show some wonders to Satyavrata. But Śrī Jīva Gosvāmī has given definite proofs from authoritative scriptures (like Viṣṇu-dharmottara, Mārkaṇḍeya Purāṇa, Harivaṁśa, etc.) that there is always a devastation after the end of each and every Manu. Śrīla Viśvanātha Cakravartī has also supported Śrīla Jīva Gosvāmī, and he (Śrī Cakravartī) has also quoted from Bhāgavatāmṛta about this inundation after each Manu. Apart from this, the Lord, in order to show special favor to Satyavrata, a devotee of the Lord, in this particular period, incarnated Himself.