Daily Reading: Srimad Bhagavatam page-62

SB 1.3.15

rūpaṁ sa jagṛhe mātsyaṁ
 cākṣuṣodadhi-samplave
nāvy āropya mahī-mayyām
 apād vaivasvataṁ manum

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.

SB 1.3.16

surāsurāṇām udadhiṁ
 mathnatāṁ mandarācalam
dadhre kamaṭha-rūpeṇa
 pṛṣṭha ekādaśe vibhuḥ

Translation: 

The eleventh incarnation of the Lord took the form of a tortoise whose shell served as a pivot for the Mandarācala Hill, which was being used as a churning rod by the theists and atheists of the universe.

Purport: 

Once both the atheists and the theists were engaged in producing nectar from the sea so that all of them could become deathless by drinking it. At that time the Mandarācala Hill was used as the churning rod, and the shell of Lord Tortoise, the incarnation of Godhead, became the resting place (pivot) of the hill in the seawater.