Why we make clay Ganesha and immerse it in the sea during Ganesha Chathurthi ?
Ganesh Chaturthi celebrations end with the immersion or Visarjan of clay Ganesha murthi in water. Hindus worship the formless ParaBrahman. But the vast majority of people need a form of the ParaBrahman to pray to, to seek help and to receive blessings.
Clay mixed with water gives form to the formlessness. During the Ganesha Chathurthi celebrations, this form is worshipped. After the celebrations, it is time to accept the eternal cosmic law that which took form has to become formless again in a never ending cycle.
Each year Ganesha arrives to teach us that forms change but the Supreme Truth remains the same. Body perishes but Brahman residing in it remains constant. This body becomes energy for another but the source of energy is the same. Bliss is achieved when we realize this.
Source: Hindu Temple Of North Texas Newsletter