Looking at the spread of humanity out of Africa, there are two scenarios which we can envision regarding population of India – hugging the coastline and reaching India by land, a water based migration towards Australia with a potential stopping in India. Due to the continuous stretch of desert from Arabia till India, is it possible that the main influx into India came through water and not land and then spread all over India? Well, whether it happened or not, I don’t know, but I don’t see any constraints in using only water to spread all over India, for that sake, all over India, China and Indo-China solely using waterways. Since there is no land component involved, the availability of proofs of this happening in itself is very hard. Have a look at this image.
This is a riverine map of Central India in the Amarkantak area.First circle gives cross over points from Narmada into Yamuna. Second, Narmada-Tapi-Godavari. Third, Narmada-Son(Ganga)-Mahanadi. Fourth, Godavari-Mahanadi. Looking at the map, there are sufficient number of points less than 50 km apart from where people can jump from one river into another. And the implications of this. Moving from Narmada into Godavari-Krishna-Kaveri belt or Narmada/Mahanadi-Ganga basin there by linking to Brahmaputra or Indus is not a big deal. Extend it a bit further, you hit Irrawady, Mekong and Yangtze and even Amu Darya – meaning the whole of Asia below Russia and east of Himalayas.
Realistically, this looks much saner than the crackpot theory of Aryan Invasion where a bunch of barbarians on horses destroy a complete civilization – all built around a few skeletons found in Mohenjodaro on the roads and the word Purandara – Destroyer of Cities with the height of irony being that a village near Kabul or Quetta is attacked and the person staying near Delhi ran away from his home, god only knowing how he got the news that fast, forget the news of destruction of Harappa or Mohenjodaro. But, did it happen, I don’t know.