The oldest Indian flood narrative is short and domestic. Manu is washing his hands and a small fish comes into the water, asking to be kept and promising to save him. He rears it in a jar, then a ditch, then the Ganges, then the sea, and it warns him of the year the flood will come and tells him to build a ship. When the water rises he ties the rope to the fish's horn and is towed to the northern mountain, where he makes the ship fast to a tree and comes down as the water falls.
What follows is the part that later tellings drop. Manu is alone. He performs a sacrifice, pouring an offering of clarified butter, curds, whey and milk into the water, and from the offering a woman comes up, and she is Ida. Through her he begets the race, so that the human line begins not from a rescued family but from a rite correctly performed by a single survivor.
The Puranic retelling reorganises all of this. The fish becomes explicitly the Matsya avatar of Vishnu, the seven great seers come aboard, the seeds of all plants are taken along, and the ship is bound with the serpent Shesha, so that the story is now about preserving a world rather than restarting one. Both versions are current in Hindu tradition, and the difference between them is a good measure of the distance between Vedic ritual thinking and Puranic devotion: in the first, a man survives by knowing how to make an offering, and in the second, a god arrives to make sure he does.