rupam sa jagrhe matsyam
navy aropya mahi-mayyam
apad vaivasvatam manum
When there was a complete inundation after the period of the Caksusa 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.
According to Sripada Sridhara Svami, the original commentator on the Bhagavatam, there is not always a devastation after the change of every Manu. And yet this inundation after the period of Caksusa Manu took place in order to show some wonders to Satyavrata. But Sri Jiva Gosvami has given definite proofs from authoritative scriptures (like Visnu-dharmottara, Markandeya Purana, Harivamsa, etc.) that there is always a devastation after the end of each and every Manu. Srila Visvanatha Cakravarti has also supported Srila Jiva Gosvami, and he (Sri Cakravarti) has also quoted from Bhagavatamrta 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.