Question: why ganesh idols are immersed in water?
Answer: The ritual is done to signify the birth cycle of Lord Ganesha; just as he was created from clay/Earth, his symbolic statue is as well. The idol is immersed in water so that Ganesha may return to his home after his 'stay' at the devotees' home or temple where the Ganesha Chaturthi rituals are conducted.