What would happen to motor if water was added with malisious intent to gas tank?

We have experienced coolent mixed into the oil, and an aboundance of white smoke from exhaust system !

A head gasket problem possibly.

If water were added to gas tank how would I diagnoses this?

Lets just say i was gona buy a 1995 ford taurus for 0 that has an engine knock i know i should not buy it but lets just say i already did. Theres a slight knock that eases once the engine is warmed up and there is no white smoke. What can cause those knocks. actually its more like a tick.