> It doesn't sound expensive to add water injection to an internal combustion engine, until you have to make it work in production, with end users.

Water / methanol injection was commonly used on various piston engine aircraft in the 1940's. It had several benefits, cooling the input air, preventing pre-detonation at high compression (due to lower octane fuel in some cases), lowering the combustion temperature, and such. It was practical and worked fine for high-performance aircraft, that saw regular maintenance after every flight.

It doesn't seem very necessary or practical for a ground vehicle though.