Aerosol car spray paint (whether sprayed by a spray gun or straight from spray cans) does not dissolve in water. The paint is made to stick and stay on the surface of the object you are painting, such as a car. Water can soften and wash away some types of paints, but it will not dissolve aerosol car spray paint because it dries quickly.
Aerosol paints dry extremely rapidly on vehicles. Once they are dry they cannot be removed by water or any other solvent that does not contain strong chemicals like acetone.
How to remove spray paint from a car
It is possible to use acetone to remove dried aerosol car spray paint, but this requires allowing the acetone to soak into the aerosol paint at least overnight before trying to wipe or scrape it off.
Acetone is like nail polish remover which is mostly composed of acetone. If you need to remove dried aerosol car spray paint from a surface, use a plastic scraper so you don't scratch the paint.
Removing spray paint from yourself
Aerosol paints (as mentioned above) are designed to be durable and not easy to remove - hence why they are supplied in protective coatings. If you do get aerosol paint on your hands, it is unlikely that water will completely remove most spray paints.
You should use soap and warm water instead of just water to help break up the dried paint so it flushes away with washing. Or you could try using dish-washing liquid or borax powder (add some water first), which should help bind the surface when washing off in the shower.