As a very rough approximation (just to get a proper order of magnitude), to make enough hydrogen fuel through electrolysis to run a car, you need about as much

As a very rough approximation (just to get a proper order of magnitude), to make enough hydrogen fuel through electrolysis to run a car, you need about as much water as you would need gasoline. Compared to daily household needs, it’s insubstantial, and compared to agricultural needs, it’s less than rounding error.