The high voltage potential is between the HV wiring harness and chassis. Not between the car and earth ground. Unless you had one hand on the car and another in the water stream the chance of getting an electric shock while wheeling a fire hose are about zilch. Even with your hand in the water stream your chances would be near zero. Clean water is a rather poor conductor. Maybe, if you stood in the puddle and put one hand on the chassis and one hand ...