Jai Sri Ganesha,
Sacred Saawan is the month that relates to the truth of water in sanctity with,
The prime cause of the water is lord Vishnu, a great devotee and unalloyed lover of Lord Shiva,
Water is offered on Shivlingam by the men in devotion with that to enjoin the austere penance being performed by lord Vishnu on the occasion,
Sri Ganga Ji was the great servant to serve the lotus feet of Lord Vishnu that redirected to Lord Shiva for a cause to serve the Yajna being performed, hence Ganga Jal is mostly offered on Shivlingam to enjoin the oblations being performed by Sri Hari Vishnu,
May Lord bless all,
Thanks, please