Water injection by itself can create additional horsepower. The expansion of the water as it suddenly becomes steam generates a lot of pressure. Aircraft jet engines with water injection (called "water wagons") have a tremendous boost as long as the water lasts. One pilot told me that the drop in power is so dramatic that you think that the engine has stalled. Such systems help with high load take offs from high altitude airports.
Water injection in a car engine is very hard to regulate and difficult to meter in enough to make very big power gains. Often it does more to cool the intake mixture (making the mixture denser similar to the effect of an intercooler) as well as cutting engine ping.
The 62 Oldsmobile F-85 with optional turbo had water/alcohol injection to prevent the engine from pinging when it was under boost. The waste gate would open and not allow boost unless there was fluid in the bottle.
Several years ago there was some study about eliminating EGR valves by using water injection to keep the combustion temperatures below the level that generates NOX. It worked, but it just was not practical.
Overall, there has really never been enough gain from water injection to warrant the developmental costs to optimize it. Nitrous injection is much more controlable and provides so much more power because of the additional oxygen that it provides.
Water injection can also cause accelerated cylinder and ring wear when used in heavy doses as well as problems with engine oil if it doesn't have special additives to deal with the additional water that can end up in the crankcase.
Like a lot of things, a little may be a good thing, but a lot may be more trouble than it is worth.
