Even if you're using water, you'll need to tune for it. If you simply connect it and drive, the water will still assist avoid explosion, but your car will have a tiny "bog" while the water is spraying. To get rid of it, you'll need to lean it out. This is done by temporarily disconnecting one of the injector circuits, so only one side of the engine gets fuel. When you reconnect both circuits, the bog will go away because now each side of the engine receives equal amounts of fuel.
This process needs to be done with care because if you break something when tuning your car, you could end up having to replace the entire engine!
The best way to do this test is to turn off the ignition and see how many times you have to push down on the accelerator before the engine catches fire. If you can keep it under 10 pushes then you're good to go.
If you time it right, you should only need to push the button once. If you wait too long after reconnecting both circuits, you might need to do it again because not all components react at the same speed. For example, some cables can become hot during operation which would cause them to feel stiff or hard to move when reconnected. Such items must be given time to cool down before trying again.
Overall, water injection is useful for increasing performance but also dangerous if not done properly.
Spraying the engine bay with water is totally safe for the vast majority of modern vehicles. Covered air boxes and weather-proof electrical connectors are standard in today's automobiles. High-pressure water should not be sprayed on the alternator, intake, or sensors.
If you own a classic car or truck, there may be parts such as the engine block or transmission case that can't withstand the heat of water. Before working on a hot engine, please check with a reputable repair shop for warnings about possible heat damage. They will be able to advise you on what needs to be done to prevent further damage.
If you are looking at old photos or drawings of cars from the 1930s through 1950s, you might see pictures of engines covered with water to remove excess heat. This was usually only done when the temperature outside reached unsafe levels or when the vehicle was being tested by automobile manufacturers before they approved it for sale. Covering the engine with water was too restrictive of airflow and could cause damage to other components.
Today's vehicles are designed with safety as their number one priority. The drivetrain is where power is transmitted from the motor to the wheels, so any problems with the transmission or differential would need to be addressed before driving off the lot. Other areas of concern include the fuel system and electrical system.
Injecting water into your engine may sound insane, but it's a turbo tuner technique for massive horsepower. The water instantly evaporates into the manifold air, lowering the intake air temperature and increasing air density. This increases the amount of oxygen available to each cylinder during each cycle. The increased oxygen flow rate also decreases the need for advanced ignition systems or high-performance fuels.
The effect of injected water on engine power is instantaneous. However, the impact of this method on engine durability is significant since the extra heat from water injection can cause engine components to fail prematurely. Also, the presence of water in your engine is likely to result in corrosion which will also reduce the lifespan of other parts such as injectors, hoses, and wiring harnesses.
Water injection is legal in most countries if you have the proper license. The technology was first developed by racing teams in Japan to make more efficient use of fuel in early turbocharged engines. It's now used by some amateur builders as well as professional teams like Red Bull Racing who achieve huge gains with this technique.
However, unless you have experience working on diesel engines or are willing to invest time learning about diesel management, I would not recommend attempting water injection without professional help.