hindus pray to images of Lord Ganesha, large and small, many of them made especially for the occasion by cottage industries and street side artisans. At the beginning of pooja the Lord is invoked by chanting mantras and if you wish to keep the idols for more number of days you have to offer special daily pooja offer Naivediams, etc. as per the custom. This is one reason for immersing the idols in the nearest water body (all rivers, lakes and the sea which are sacred to Hindus).
The idea is that the Ganapathy that people worship during the Ganesh chathurthy every year is like the urchava Moorthy of the Deity in temples every year people use a new one. After the celebration, they cannot keep it forever as every year a new one would e added to the stock if they were to be kept.
Apart from the term hinduism also one can analyze about the five natural elements air, water, earth (land), fire, space. The one we can externally in touch with or realize through your five senses and make it as your part is none other than water. Think about the remaining four cosmic sources and try to experience through the five senses and make it as you it will not have completeness as like water.
