Introduction
Satyavrata is a king, in the Bhagavata Purana's account of the Matsya avatar, who is warned of an approaching cosmic deluge and saves the seeds of life aboard a boat, guided by Vishnu in the form of a fish. In some tellings he is identified with, or becomes, Vaivasvata Manu, the ancestor of the present human race and of the Solar dynasty.
Meeting the fish
While offering water at a river, Satyavrata found a tiny fish, which asked for his protection from larger fish. He kept it first in a pot, then a larger vessel, a pond, a lake and finally the ocean, as it grew to an enormous size, and realised it was Vishnu.
The flood and the boat
The fish, Matsya, told Satyavrata that in seven days the world would be destroyed by flood, and instructed him to build a boat and gather the seven great sages, along with seeds of every plant and pairs of every kind of living being. When the flood came, Matsya, now vast, towed the boat, tied by its horn to the serpent Vasuki, safely through the deluge, and taught Satyavrata sacred knowledge during the voyage.
Identification with Manu
The Bhagavata Purana names this king Satyavrata, and identifies him with Vaivasvata Manu, the seventh Manu, from whom the Ikshvaku and Solar dynasty descend; other texts, notably the earlier Shatapatha Brahmana, tell essentially the same flood story with Manu as the central figure from the outset, without the separate name Satyavrata.