You should realise their mistake by telling that telling the importance of water.
there is 75% water present on earth. but that water is saline water. the water which is not saline or fresh water is only 2% in India. if we waste water we will suffer a lot in future. water is used for many purpose. not only for washing car and courtyard. you should also tell them about conservation of water like rainwater harvesting.
you should guide them about water management system and how to use water in proper way. you should do campaign for water conservation so that people become aware.
Offer to wash his/her car and show how to do it with minimum water. Actions always speak louder than words!