In the 19 century it was admitted that India was the mother of the civilization but in fact the big civilization could be only the middle east and Egypt.
Mortimer Willer who was one of the greatest British archaeologist who worked in India called it the Cinderella of the civilization because it was the little last one, we even thought that it was a pale copy of Mesopotamian civilization before to see that it was a totally different civilization.
When we start to search we saw that there was no material which can be similar to Mesopotamian civilization, but we saw very quickly that there was huge town, there are sites which are more than 200-300 hectare
Mohenjodaro is probably the biggest town of all the antic town.
The big watercourses like the Indus, are very important in the developpment of this civilization like each agricultural civilization, it's because of the watercourses that it's possible to have agriculture to feed towns which where the population will be less paysant but more laborer and craftsman, long time ago the Indus spread everywhere on a flat plain, and it was on this flat plain that we can find agriculture with the risk to see each year the field disappear (flooded) .
We have a precise idea of the power of the economic system in the big towns, particulary in Mohenjo Daro or Harada, we have extremely powerful craft that produce product that only this civilization know how to do it as for example a type of ceramic almost industrial, without counting all what disappeared, the Indus population was probably a huge textile's manufacturer, a huge processed food product with their era techniques, all that we do not have any traces, all that have disappeared but over all we can say that in the Indus civilization some product had a status that we can nowadays qualify as industrial and was exported everywhere even until Mesopotamia.
One of the problems of the Indus civilization was that those people was writing, we know that, we have thousands and thousands of "documents" but they was writing on a perishable medium, so we didn't find the real writing, what we found was like nowadays the name of the street on plates, names/indications on key-holder, etc ... we found everything that stay in times, but the perishable medium disappeared so we don't know, we do not have full text.
So we saw a civilization that was writing, but we didn't know, we thought that it was a kind of magic writing, but in fact it was probably a civilization which have politic and administrative government like Egypt.
For sure until we will be able to decrypt the writing , meaning to find a text long enough to apply the classic techniques of decryption, we even do not know nowadays what kind of language those people was using.
Sea transportation for commercial purpose of the Indus civilization is a question that we start around 20 years ago. we knew that the rivers was used for the commerce between Indus towns, the question about the sea commercial transportation is more sensitive. we know materials from Indus, it come from the Oman's peninsula and some countries around, but more important than that we have Mesopotamian's texts around 2300BC saying that came to tie up to the dock of his capital Guilmun (a zone between the Kuwait and Quatar) some boats from "Magan" some boats from "Melhunra" (not sure about the name's spelling) which is the country of the Indus.
Probably it was a commerce with all the people of this area.
Also probably, It was not the people from the Indus which start to navigate on the sea, because they was lands people and you can imagine that all the Indus area (the Delta of the river) was a marsh area, a difficult area to pass through, but after a wile, in the middle of the third millennium (BC) this area is an international commercial area, where pass jewelries like the big pearl that only people of Indus manufacture and are the only one to know how to manufacture it, but also the commerce is always about textiles and everything from everywhere like ivory combs etc ..
How they communicate when they was trading ? we have a Sumerian text at the end of the third millennium in Mesopotamia which "speak" about a "Malhoura" translator, a guy who know Indus language and Sumerian language.
It's a shame that this guy didn't let us a dictionary (the french guy is joking).
But in fact all this pre-historic world, where people was always trading is a multi-language world, it's only in our world where people speak one language plus the English.
How they disappear (Indus civilization)? roughly he said (i'm a bit tired) the government became more local, in the big towns they start to live differently which by no means it mean they live it, they just live differently and he said that in archeology it's hard to see it, so they do not really know.