I have a degree in cultural anthropology and have taken a couple of courses that touched on this.
If I remember correctly and didn't kill these brain cells yet, the Ganges River in Hinduism is sacred and is seen as a manifestation of the goddess Ganga(?). Contact with the river (eww!) helps to wash away sin or bad karma and facilitates the release from samsara, or the cycle of life and death. Basically, they believe that if you are reincarnated, your karma wasn't so hot and you have to suffer a life all over again. The trick is to be pure and break free. The river can help you with this, floating stiffs and all. Tossing a dead relative in the river can't hurt, right?