"For if you compared it to the ways in which the dolphin's echoes mirrored the world, speech was only a bag of tricks that fooled you into believing that you could see through the eyes of another being."
This quote from the Hungry Tide, increased my curiosity about what was so special about a dolphin's echo. After googling it I found that there is a phenomenon called Dolphin's Echolocation ( the location of an object by its echoes). A Dolphin can locate it prey using her echoes. Pretty neat, hence her echoes are nothing but a bag of tricks.
You can find more information here http://www.inkokomo.com/dolphin/echolocation.html
And from this video below from YouTube, it was evident that Piya in the book for searching for Orcella dolphins in the Sundarbans - where the ganges breakdown into tributaries and join the bay of bengal. And in this video the researchers too are referring to the special breed of dolphins found in the ganges.