"One day when the sage Manu was performing his ablutions, he found in the hollow of his hand a tiny fish which begged him to spare its life. He put it in a jar, but in one day the fish grew so big that he had to put it in a tank: The next day, it needed a lake. Soon the lake was too small: 'Throw me into the sea,' said the fish. 'I shall be better there.' Then he warned manu of an approaching flood. He sent him a large craft, and ordered him to take on board with him a pair of each living species and the seeds of all plants.

Almost before Manu could obey, the world was flooded: Only Vishnu remained visible in the shape of a great sea-unicorn with scales of gold. Manu anchored his vessel to the fish's horn, with the great serpent Vasuki serving as a rope. And thus humanity, animals and plants were saved from destruction.

...........World Mythology, p 212.

"The Earth will be submerged in water with all its mountains and trees and houses. This boat has been fashioned out of the assemblage of all the gods in order to protect the assemblage of great living souls, of Lord of the Earth. Those born of sweat, those born of eggs, or of water and those living creatures which slough their skins--place them all in this boat and save them, for they have no protector. And when your ship is struck by the winds that blow at the end of the age, fasten the ship to this horn of mine, O King, Lord of Kings, Lord of the Earth. At the end of the dissolution, you will the the Prajapati of the whole universe, moving and still. Thus at the beginning of the Krta Age, you will the the omniscient, firm king, the overlord of the period of Manu, worshipped even by the gods."

.............Hindu Myths, p 183.